Derivative Financial Instruments - CDS notional amounts by credit rating and fair value of protection sold (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	$ 4,690	$ 3,388
Credit default contracts
Credit Derivatives
Notional amount	265	105
Fair value	4	1
Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	15	25
Fair value	0	0
Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	250	80
Fair value	4	1
AA | Credit default contracts
Credit Derivatives
Notional amount	4	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	4	1
A | Credit default contracts
Credit Derivatives
Notional amount	44	29
A | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	0	10
A | Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	44	19
BBB | Credit default contracts
Credit Derivatives
Notional amount	186	55
BBB | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	10	10
BBB | Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	176	45
BB and lower | Credit default contracts
Credit Derivatives
Notional amount	31	20
BB and lower | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	5	5
BB and lower | Credit default contracts | Index | Corporate debt
Credit Derivatives
Notional amount	$ 26	$ 15