NOTES TO CONSOLIDATED FINANCIAL STATEMENTS OF AMERICAN AIRLINES GROUP INC. 16. Valuation and Qualifying Accounts (in millions) 126 Balance at Beginning of Year Additions Charged to Statement of Operations Accounts Deductions Balance at End of Year Allowance for obsolescence of spare parts Year ended December 31, 2018 $ 769 $ 70 $ (25) $ 814 Year ended December 31, 2017 765 29 (25 ) 769 Year ended December 31, 2016 728 37 — 765 Allowance for uncollectible accounts Year ended December 31, 2018 $ 24 $ 42 $ (37) $ 29 Year ended December 31, 2017 36 43 (55 ) 24 Year ended December 31, 2016 41 47 (52) 36 Stock-settled RSU award activity for all plans for the years ended December 31, 2018, 2017 and 2016 is as follows: As of December 31, 2018, there was $111 million of unrecognized compensation cost related to stock-settled RSUs. These costs are expected to be recognized over a weighted average period of one year. The total fair value of stock-settled RSUs vested during the years ended December 31, 2018, 2017 and 2016 was $91 million, $123 million and $107 million, respectively. Number of Shares Weighted Average Grant Date Fair Value (In thousands) Outstanding at December 31, 2015 5,607 $ 38.08 Granted 2,655 41.34 Vested and released (2,754 ) 34.83 Forfeited (321) 40.15 Outstanding at December 31, 2016 5,187 $ 41.48 Granted 2,309 48.58 Vested and released (2,708 ) 39.63 Forfeited (464) 44.48 Outstanding at December 31, 2017 4,324 $ 46.94 Granted 2,194 47.65 Vested and released (1,999 ) 44.99 Forfeited (199) 45.72 Outstanding at December 31, 2018 4,320 $ 44.29