Operating Special Items, Net 2018 2017 (In millions) Fleet restructuring expenses  (1) $ 422 $ 232 Merger integration expenses  (2) 268 273 Severance expenses  (3) 58 — Litigation settlement  (4) 45 — Intangible asset impairment  (5) 26 — Labor contract expenses 13 46 Mark-to-market adjustments on bankruptcy obligations, net  (6) (76 ) 27 Employee 2017 Tax Act bonus expense  (7) — 123 Other operating charges, net 31 11 Total mainline operating special items, net 787 712 Regional operating special items, net — 3 Total operating special items, net $ 787 $ 715 Year Ended December 31, $^{(1)}$  Fleet restructuring expenses principally included accelerated depreciation and rent expense for aircraft and related equipment grounded or expected to be grounded earlier than planned. $^{(2)}$  Merger integration expenses included costs associated with integration projects, principally American's flight attendant, human resources and payroll, and technical operations systems. $^{(3)}$  Severance expenses primarily included costs associated with reductions of management and support staff team members. $^{(4)}$  Settlement of a private party antitrust lawsuit. See Note 10(e) - “ Private Party Antitrust Action Related to Passenger Capacity ” to American’s Consolidated Financial Statements in Part II, Item 8B for further discussion. $^{(5)}$  Intangible asset impairment includes a non-cash charge to write-off American’s Brazil route authority as a result of the U.S.-Brazil open skies agreement. $^{(6)}$  Bankruptcy obligations that will be settled in shares of AAG common stock are marked-to-market based on AAG’s stock price. $^{(7)}$  Employee bonus expense included costs related to the $1,000 cash bonus and associated payroll taxes granted to mainline employees as of December 31, 2017 in recognition of the 2017 Tax Act. American’s short-term investments in each period consisted of highly liquid investments that provided nominal returns. Interest income increased $115 million, or 53.4%, due to higher interest-bearing related party receivables from American’s parent company, AAG, as well as a 103 basis point increase in average yields in 2018 as compared to 2017. In 2018, other nonoperating income, net principally included $309 million of non-service related pension and other postretirement benefit plan income, which reflects an increase in the expected return on pension plan assets in 2018 as Nonoperating Results Year Ended December 31, Increase (Decrease) Percent Increase (Decrease) 2018 2017 (In millions, except percentage changes) Interest income $ 330 $ 215 $ 115 53.4 Interest expense, net (1,028) (988) (40) 4.1 Other income, net 167 123 44 35.3 Total nonoperating expense, net $ (531) $ (650) $ 119 (18.2) 61