Derivative Financial Instruments and Off-balance sheet Financial Instruments - Counterparty credit exposure and other (Details) $ in Millions	Dec. 31, 2017 USD ($) counter-party	Dec. 31, 2016 USD ($) counter-party
Credit Derivatives
Cash and securities pledged as collateral by counterparties	$ 3
Securities pledged as collateral to counterparties	4
Collateral posted under MNAs for contracts containing credit-risk-contingent features	3	$ 0
Collateral already posted, aggregate fair value, without credit-risk-contingent features	1
Gross liability fair value of contracts containing credit-risk-contingent features	12	2
Gross asset fair value of contracts containing credit-risk-contingent features and subject to MNAs	(5)	(2)
Collateral posted under MNAs for contracts containing credit-risk-contingent features	(3)	0
Maximum amount of additional exposure for contracts with credit-risk-contingent features if all features were triggered concurrently	$ 4	$ 0
A plus
Credit Derivatives
Number of counter-parties | counter-party	2	5
Notional amount	$ 59	$ 312
Credit exposure	3	12
Exposure, net of collateral	$ 0	$ 9