Derivative Financial Instruments - CDS Notional Amounts by Credit Rating (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 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	$ 2,482	$ 2,626
Credit default contracts
Credit Derivatives
Notional amount	1	81
Fair value	0	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		80
Fair value		1
AA | Credit default contracts
Credit Derivatives
Notional amount	0	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
A | Credit default contracts
Credit Derivatives
Notional amount	0	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		19
BBB | Credit default contracts
Credit Derivatives
Notional amount	0	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		45
BB and lower | Credit default contracts
Credit Derivatives
Notional amount	1	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		$ 15