Derivative Financial Instruments and Off-balance sheet Financial Instruments - CDS notional amounts by credit rating (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) entity	Dec. 31, 2018 USD ($)
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Term of credit default swaps (in years)	5 years
Credit Derivatives
Notional amount	$ 2,385	$ 2,399
The number of reference entities generally included in a CDX index | entity	125
Credit default contracts
Credit Derivatives
Notional amount	$ 5	1
Fair value	0	0
Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	5	1
Fair value	0	0
AA | Credit default contracts
Credit Derivatives
Notional amount	0	0
AA | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	0	0
A | Credit default contracts
Credit Derivatives
Notional amount	0	0
A | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	0	0
BBB | Credit default contracts
Credit Derivatives
Notional amount	0	0
BBB | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	0	0
BB and lower | Credit default contracts
Credit Derivatives
Notional amount	5	1
BB and lower | Credit default contracts | Single name | Corporate debt
Credit Derivatives
Notional amount	$ 5	$ 1