Derivative Financial Instruments - OTC derivatives counterparty credit exposure by counterparty credit rating (Details) $ in Millions	Jun. 30, 2018 USD ($) counter-party	Dec. 31, 2017 USD ($) counter-party
Credit Derivatives
Number of counter- parties | counter-party	5	4
Notional amount	$ 1,762	$ 108
Credit exposure	16	4
Exposure, net of collateral	$ 1	$ 1
AA-
Credit Derivatives
Number of counter- parties | counter-party	0	1
Notional amount	$ 0	$ 18
Credit exposure	0	1
Exposure, net of collateral	$ 0	$ 0
A plus
Credit Derivatives
Number of counter- parties | counter-party	0	3
Notional amount	$ 0	$ 90
Credit exposure	0	3
Exposure, net of collateral	$ 0	$ 1
A-
Credit Derivatives
Number of counter- parties | counter-party	2	0
Notional amount	$ 34	$ 0
Credit exposure	2	0
Exposure, net of collateral	$ 0	$ 0
BBB plus
Credit Derivatives
Number of counter- parties | counter-party	2	0
Notional amount	$ 1,718	$ 0
Credit exposure	14	0
Exposure, net of collateral	$ 1	$ 0
BBB
Credit Derivatives
Number of counter- parties | counter-party	1	0
Notional amount	$ 10	$ 0
Credit exposure	0	0
Exposure, net of collateral	$ 0	$ 0