Derivative Financial Instruments - OTC derivatives counterparty credit exposure by counterparty credit rating (Details) $ in Millions	Jun. 30, 2019 USD ($) counter-party	Dec. 31, 2018 USD ($) counter-party
Credit Derivatives
Number of counter- parties | counter-party	7	5
Notional amount	$ 1,202	$ 764
Credit exposure	34	20
Exposure, net of collateral	$ 4	$ 1
A plus
Credit Derivatives
Number of counter- parties | counter-party	6	3
Notional amount	$ 1,121	$ 643
Credit exposure	32	19
Exposure, net of collateral	$ 4	$ 1
A
Credit Derivatives
Number of counter- parties | counter-party	1	2
Notional amount	$ 81	$ 121
Credit exposure	2	1
Exposure, net of collateral	$ 0	$ 0