Employee Benefits - Derivative Instruments (Details) - Combined Pension Plan - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Exchange-traded U.S. equity futures
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	$ 84	$ 184
Exchange-traded Treasury and other interest rate futures
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	1,033	1,253
Exchange-traded Foreign currency futures
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	12	0
Exchange-traded EURO futures
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	6	10
Interest rate swaps
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	124	44
Credit default swaps
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	43	205
Index swaps
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	1,297	2,058
Foreign exchange forwards
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	769	508
Options
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	$ 222	$ 146