Derivative Financial Instruments - CDS Notional Amounts by Credit Rating (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Term of credit default swaps (in years)	5 years
Credit Derivatives
Notional amount	$ 2,515	$ 2,626
Credit default contracts
Credit Derivatives
Notional amount	31	81
Fair value	1	1
Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	1	1
Fair value	0	0
Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	30	80
Fair value	1	1
AA | Credit default contracts
Credit Derivatives
Notional amount	1	1
AA | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	0	0
AA | Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	1	1
A | Credit default contracts
Credit Derivatives
Notional amount	5	19
A | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	0	0
A | Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	5	19
BBB | Credit default contracts
Credit Derivatives
Notional amount	21	45
BBB | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	0	0
BBB | Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	21	45
BB and lower | Credit default contracts
Credit Derivatives
Notional amount	4	16
BB and lower | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	1	1
BB and lower | Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	$ 3	$ 15