Derivative Financial Instruments - Derivative Netting (Details) $ in Millions	Jun. 30, 2018 USD ($) party	Dec. 31, 2017 USD ($) party
Credit Derivatives
Number of counter-parties (in Party) | party	4	2
Notional amount	$ 266	$ 59
Credit exposure	2	3
Exposure, net of collateral	$ 0	$ 0
A plus
Credit Derivatives
Number of counter-parties (in Party) | party	0	2
Notional amount	$ 0	$ 59
Credit exposure	0	3
Exposure, net of collateral	$ 0	$ 0
A-
Credit Derivatives
Number of counter-parties (in Party) | party	2	0
Notional amount	$ 33	$ 0
Credit exposure	1	0
Exposure, net of collateral	$ 0	$ 0
BBB plus
Credit Derivatives
Number of counter-parties (in Party) | party	1	0
Notional amount	$ 231	$ 0
Credit exposure	1	0
Exposure, net of collateral	$ 0	$ 0
BBB
Credit Derivatives
Number of counter-parties (in Party) | party	1	0
Notional amount	$ 2	$ 0
Credit exposure	0	0
Exposure, net of collateral	$ 0	$ 0