Derivative Financial Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Offsetting Assets and liabilities [Line Items]
Asset Fair Value	$ 136	$ 471
Liability Fair Value	85	173
Counterparty netting, Assets	(50)	(169)
Counterparty netting, Liabilities	(50)	(169)
Cash collateral netting, Assets	(84)	(266)
Cash collateral netting, Liabilities	0	0
Securities collateral netting, Assets	0	(11)
Securities collateral netting, Liabilities	(30)	(2)
Net receivables	2	25
Net payables	5	2
Equity contract
Offsetting Assets and liabilities [Line Items]
Notional Amount	154	89
Asset Fair Value	9	2
Liability Fair Value	7	0
Foreign exchange contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	559	406
Asset Fair Value	0	25
Liability Fair Value	52	19
Interest rate contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	17,286	25,606
Asset Fair Value	117	438
Liability Fair Value	20	154
Credit contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	771	426
Asset Fair Value	10	6
Liability Fair Value	$ 6	$ 0