Employee Benefits - Derivative Instruments (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Exchange Traded Domestic Equity Futures
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	$ 300	$ 256
Exchange Traded Treasury Futures
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	3,901	1,830
Interest Rate Swap
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	83	137
Credit Default Swap
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	66	100
Equity Swap
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	0	1
Foreign Exchange Forward
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	295	293
Options Held
Defined Benefit Plan Disclosure [Line Items]
Derivative, notional amount	$ 192	$ 259