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Employee Benefit Plans - Components of Net Periodic Benefit Cost (Benefit) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Pension Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 1 $ 1 $ 3 $ 3
Interest cost (24) (31) (71) (95)
Expected return on plan assets (51) (53) (153) (161)
Amortization of prior service credit 0 0 0 0
Amortization of actuarial loss (17) (15) (52) (45)
Net periodic cost (benefit) (9) (6) (27) (18)
Other Postretirement Benefits Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0 0 0 0
Interest cost 0 (1) (2) (4)
Expected return on plan assets 0 0 (2) (2)
Amortization of prior service credit (2) (2) (5) (5)
Amortization of actuarial loss (2) (2) (6) (5)
Net periodic cost (benefit) $ 0 $ 1 $ 1 $ 2