Derivative Financial Instruments and Off-balance sheet Financial Instruments - Gross and net amounts for OTC derivatives (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Asset derivatives
Gross amount	$ 127	$ 121
Derivative asset, fair value, gross liability	(1)	(28)
Net amount on balance sheet	126	93
Liability derivatives
Gross amount	(369)	(334)
Liability derivatives, Gross asset	6	0
Net amount on balance sheet	(363)	(334)
OTC derivatives
Asset derivatives
Gross amount	8	31
Derivative asset, fair value, gross liability	(7)	(28)
Derivative asset offsets under cash collateral pledged	1	19
Net amount on balance sheet	2	22
Securities collateral (received) pledged	0	(9)
Net amount	2	13
Liability derivatives
Gross amount	(26)	(33)
Liability derivatives, Gross asset	7	28
Derivative liability offsets under cash collateral pledged	7	0
Net amount on balance sheet	(12)	(5)
Securities collateral (received) pledged	3	4
Net amount	$ (9)	$ (1)