Derivative Financial Instruments - OTC derivatives counterparty credit exposure by counterparty credit rating (Details) $ in Millions	Mar. 31, 2018 USD ($) counter-party	Dec. 31, 2017 USD ($) counter-party
Credit Derivatives
Number of counter- parties | counter-party	3	4
Notional amount	$ 69	$ 108
Credit exposure	8	4
Exposure, net of collateral	$ 0	$ 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	3	3
Notional amount	$ 69	$ 90
Credit exposure	8	3
Exposure, net of collateral	$ 0	$ 1