Derivative Financial Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Offsetting Assets and liabilities [Line Items]
Asset Fair Value	$ 192	$ 136
Liability Fair Value	85	85
Counterparty netting, Assets	(51)	(50)
Counterparty netting, Liabilities	(51)	(50)
Cash collateral netting, Assets	(137)	(84)
Cash collateral netting, Liabilities	0	0
Securities collateral netting, Assets	0	0
Securities collateral netting, Liabilities	(33)	(30)
Net receivables	4	2
Net payables	1	5
Credit contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	627	771
Asset Fair Value	9	10
Liability Fair Value	3	6
Equity contract
Offsetting Assets and liabilities [Line Items]
Notional Amount	103	154
Asset Fair Value	5	9
Liability Fair Value	4	7
Foreign exchange contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	594	559
Asset Fair Value	2	0
Liability Fair Value	40	52
Interest rate contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	17,577	17,286
Asset Fair Value	176	117
Liability Fair Value	$ 38	$ 20