Derivative Financial Instruments - OTC derivatives counterparty credit exposure by counterparty credit rating (Details) $ in Millions	Mar. 31, 2019 USD ($) counter-party	Dec. 31, 2018 USD ($) counter-party
Credit Derivatives
Number of counter- parties | counter-party	5	5
Notional amount	$ 864	$ 764
Credit exposure	22	20
Exposure, net of collateral	$ 2	$ 1
A plus
Credit Derivatives
Number of counter- parties | counter-party	4	3
Notional amount	$ 748	$ 643
Credit exposure	21	19
Exposure, net of collateral	$ 1	$ 1
A
Credit Derivatives
Number of counter- parties | counter-party	1	2
Notional amount	$ 116	$ 121
Credit exposure	1	1
Exposure, net of collateral	$ 1	$ 0