Derivative Financial Instruments - OTC derivatives counterparty credit exposure by counterparty credit rating (Details) $ in Millions	Mar. 31, 2020 USD ($) counter-party	Dec. 31, 2019 USD ($) counter-party
Credit Derivatives
Number of counter- parties | counter-party	5	6
Notional amount	$ 939	$ 868
Credit exposure	46	29
Exposure, net of collateral	$ 2	$ 0
A plus
Credit Derivatives
Number of counter- parties | counter-party	4	6
Notional amount	$ 718	$ 868
Credit exposure	40	29
Exposure, net of collateral	$ 2	$ 0
A
Credit Derivatives
Number of counter- parties | counter-party	1	0
Notional amount	$ 221	$ 0
Credit exposure	6	0
Exposure, net of collateral	$ 0	$ 0