Derivative Financial Instruments and Off-balance sheet Financial Instruments - OTC derivatives counterparty credit exposure (Details) $ in Millions	Dec. 31, 2017 USD ($) counter-party	Dec. 31, 2016 USD ($) counter-party
Derivative Financial Instruments and Off-balance sheet Financial Instruments
Cash and securities pledged as collateral by counterparties	$ 3
Securities pledged as collateral to counterparties	14
Collateral posted under MNAs, credit-risk-contingent provisions in a liability position	6	$ 0
Collateral already posted, aggregate fair value, without credit-risk-contingent features	$ 8
Credit Derivatives
Number of counter-parties | counter-party	4	8
Notional amount	$ 108	$ 888
Credit exposure	4	23
Exposure, net of collateral	1	12
Gross liability fair value of contracts containing credit-risk-contingent features	28	9
Gross asset fair value of contracts containing credit-risk-contingent features and subject to MNAs	(17)	(7)
Collateral posted under MNAs for contracts containing credit-risk-contingent features	(6)	0
Maximum amount of additional exposure for contracts with credit-risk-contingent features if all features were triggered concurrently	$ 5	$ 2
AA–
Credit Derivatives
Number of counter-parties | counter-party	1	2
Notional amount	$ 18	$ 80
Credit exposure	1	2
Exposure, net of collateral	$ 0	$ 2
A plus
Credit Derivatives
Number of counter-parties | counter-party	3	5
Notional amount	$ 90	$ 698
Credit exposure	3	20
Exposure, net of collateral	$ 1	$ 9
A-
Credit Derivatives
Number of counter-parties | counter-party	0	1
Notional amount	$ 0	$ 110
Credit exposure	0	1
Exposure, net of collateral	$ 0	$ 1