Derivative Financial Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Offsetting Assets and liabilities [Line Items]
Asset Fair Value	$ 163	$ 136
Liability Fair Value	70	85
Counterparty netting, Assets	(37)	(50)
Counterparty netting, Liabilities	(37)	(50)
Cash collateral netting, Assets	(122)	(84)
Cash collateral netting, Liabilities	0	0
Securities collateral netting, Assets	0	0
Securities collateral netting, Liabilities	(30)	(30)
Net receivables	4	2
Net payables	3	5
Credit contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	639	771
Asset Fair Value	7	10
Liability Fair Value	3	6
Equity contract
Offsetting Assets and liabilities [Line Items]
Notional Amount	126	154
Asset Fair Value	5	9
Liability Fair Value	4	7
Foreign exchange contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	598	559
Asset Fair Value	3	0
Liability Fair Value	44	52
Interest rate contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	15,713	17,286
Asset Fair Value	148	117
Liability Fair Value	$ 19	$ 20