Derivative Financial Instruments - OTC derivatives counterparty credit exposure by counterparty credit rating (Details) $ in Millions	Mar. 31, 2021 USD ($) counter-party	Dec. 31, 2020 USD ($) counter-party
Credit Derivatives
Number of counter- parties | counter-party	2	1
Notional amount	$ 338	$ 186
Credit exposure	10	4
Exposure, net of collateral	$ 0	$ 0
A+
Credit Derivatives
Number of counter- parties | counter-party	1	1
Notional amount	$ 189	$ 186
Credit exposure	8	4
Exposure, net of collateral	$ 0	$ 0
A
Credit Derivatives
Number of counter- parties | counter-party	1	0
Notional amount	$ 149	$ 0
Credit exposure	2	0
Exposure, net of collateral	$ 0	$ 0