Employee Benefits - Derivative Instruments (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Exchange Traded Domestic Equity Futures
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	$ 184	$ 300
Exchange Traded Treasury Futures
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	1,253	3,901
Exchange-traded Euro futures
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	10	0
Interest Rate Swap
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	44	83
Credit Default Swap
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	205	66
Index Swap
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	2,058	0
Foreign Exchange Forward
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	508	295
Options Held
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	$ 146	$ 192