The investment strategy is to maximize return while maintaining an appropriate level of risk to provide sufficient liquidity for benefit obligations and plan expenses. During 2018, a target allocation of 97% long-duration fixed income and 3% private equity was adopted and is now maintained for the principal defined benefit pension plan, the Bristol-Myers Squibb Retirement Income Plan. BMS common stock represents less than 1% of the plan assets at December 31, 2018 and 2017. Contributions and Estimated Future Benefit Payments Contributions to pension plans were $71 million in 2018, $396 million in 2017 and $81 million in 2016 and are not expected to be material in 2019. Estimated annual future benefit payments for non-terminating plans (including lump sum payments) will be approximately $100 million in each of the next five years and in the subsequent five year period. Savings Plans The principal defined contribution plan is the Bristol-Myers Squibb Savings and Investment Program. The contribution is based on employee contributions and the level of Company match. The expense attributed to defined contribution plans in the U.S. was approximately $200 million in 2018, 2017 and 2016. On May 1, 2012, the shareholders approved the 2012 Plan, which replaced the 2007 Stock Incentive Plan. The 2012 Plan provides for 109 million shares to be authorized for grants, plus any shares from outstanding awards under the 2007 Plan as of February 29, 2012 that expire, are forfeited, canceled, or withheld to satisfy tax withholding obligations. As of December 31, 2018, 102 million shares were available for award. Shares are issued from treasury stock to satisfy our obligations under this Plan. Executive officers and key employees may be granted options to purchase common stock at no less than the market price on the date the option is granted. Options generally become exercisable ratably over four years and have a maximum term of ten years. The plan provides for the granting of stock appreciation rights whereby the grantee may surrender exercisable rights and receive common stock and/or cash measured by the excess of the market price of the common stock over the option exercise price. The Company has not granted any stock options or stock appreciation rights since 2009. Restricted stock units may be granted to key employees, subject to restrictions as to continuous employment. Generally, vesting occurs ratably over a four year period from grant date. A stock unit is a right to receive stock at the end of the specified vesting period but has no voting rights. Market share units are granted to executives. Vesting is conditioned upon continuous employment until the vesting date and a payout factor of at least 60% of the share price on the award date. The payout factor is the share price on vesting date divided by share price on award date, with a maximum of 200%. The share price used in the payout factor is calculated using an average of the closing prices on the grant or vest date, and the nine trading days immediately preceding the grant or vest date. Vesting occurs ratably over four years. Performance share units are granted to executives, have a three year cycle and are granted as a target number of units subject to adjustment. The number of shares issued when performance share units vest is determined based on the achievement of performance goals and based on the Company's three-year total shareholder return relative to a peer group of companies. Vesting is conditioned upon continuous employment and occurs on the third anniversary of the grant date. Stock-based compensation expense for awards ultimately expected to vest is recognized over the vesting period. Forfeitures are estimated based on historical experience at the time of grant and revised in subsequent periods if actual forfeitures differ from those estimates. Other information related to stock-based compensation benefits are as follows: 94 Year Ended December 31, Dollars in Millions 2018 2017 2016 Restricted stock units $ 102 $ 95 $ 89 Market share units 38 35 37 Performance share units 81 69 79 Total stock-based compensation expense $ 221 $ 199 $ 205 Income tax benefit $ 41 $ 59 $ 69 Note 17. EMPLOYEE STOCK BENEFIT PLANS