Derivative Financial Instruments and Off-balance sheet Financial Instruments - CDS notional amounts by credit rating (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) entity	Dec. 31, 2017 USD ($)
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Term of credit default swaps (in years)	5 years
Credit Derivatives
Notional amount	$ 2,399	$ 2,626
The number of reference entities generally included in a CDX index | entity	125
Credit default swaps
Credit Derivatives
Notional amount	$ 1	81
Fair value	0	1
Credit default swaps | Single name | Corporate debt
Credit Derivatives
Notional amount	1	1
Fair value	0	0
Credit default swaps | Index | Corporate debt
Credit Derivatives
Notional amount		80
Fair value		1
AA | Credit default swaps
Credit Derivatives
Notional amount	0	1
AA | Credit default swaps | Single name | Corporate debt
Credit Derivatives
Notional amount	0	0
AA | Credit default swaps | Index | Corporate debt
Credit Derivatives
Notional amount		1
A | Credit default swaps
Credit Derivatives
Notional amount	0	19
A | Credit default swaps | Single name | Corporate debt
Credit Derivatives
Notional amount	0	0
A | Credit default swaps | Index | Corporate debt
Credit Derivatives
Notional amount		19
BBB | Credit default swaps
Credit Derivatives
Notional amount	0	45
BBB | Credit default swaps | Single name | Corporate debt
Credit Derivatives
Notional amount	0	0
BBB | Credit default swaps | Index | Corporate debt
Credit Derivatives
Notional amount		45
BB and lower | Credit default swaps
Credit Derivatives
Notional amount	1	16
BB and lower | Credit default swaps | Single name | Corporate debt
Credit Derivatives
Notional amount	$ 1	1
BB and lower | Credit default swaps | Index | Corporate debt
Credit Derivatives
Notional amount		$ 15