Derivative Financial Instruments - CDS Notional Amounts by Credit Rating (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Term of credit default swaps (in years)	5 years
Credit Derivatives
Notional amount	$ 2,567	$ 2,626
Credit default contracts
Credit Derivatives
Notional amount		81
Fair value		1
Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount		1
Fair value		0
Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	80	80
Fair value	0	1
AA | Credit default contracts
Credit Derivatives
Notional amount		1
AA | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount		0
AA | Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	1	1
A | Credit default contracts
Credit Derivatives
Notional amount		19
A | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount		0
A | Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	19	19
BBB | Credit default contracts
Credit Derivatives
Notional amount		45
BBB | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount		0
BBB | Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	47	45
BB and lower | Credit default contracts
Credit Derivatives
Notional amount		16
BB and lower | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount		1
BB and lower | Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	$ 13	$ 15