Derivative Financial Instruments - OTC derivatives counterparty credit exposure by counterparty credit rating (Details) $ in Millions	Sep. 30, 2020 USD ($) counter-party	Dec. 31, 2019 USD ($) counter-party
Credit Derivatives
Number of counter- parties | counter-party	4	6
Notional amount	$ 522	$ 868
Credit exposure	17	29
Exposure, net of collateral	$ 0	$ 0
A plus
Credit Derivatives
Number of counter- parties | counter-party	3	6
Notional amount	$ 347	$ 868
Credit exposure	15	29
Exposure, net of collateral	$ 0	$ 0
A
Credit Derivatives
Number of counter- parties | counter-party	1	0
Notional amount	$ 175	$ 0
Credit exposure	2	0
Exposure, net of collateral	$ 0	$ 0