Derivative Financial Instruments - Gross and net amounts for OTC derivatives (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Asset derivatives
Asset derivatives gross amount	$ 104	$ 127
Asset derivatives, gross liability	(2)	(1)
Derivative assets net amount on balance sheet	102	126
Liability derivatives
Liability derivatives gross amount	(304)	(369)
Liability derivatives, gross asset	23	6
Derivative liabilities net amount on balance sheet	(281)	(363)
OTC derivatives
Asset derivatives
Asset derivatives gross amount	25	8
Asset derivatives, gross liability	(25)	(7)
Derivative, collateral, right to reclaim cash	2	1
Derivative assets net amount on balance sheet	2	2
Derivative assets received under securities collateral	0	0
Derivative asset, fair value, amount offset against collateral	2	2
Liability derivatives
Liability derivatives gross amount	(11)	(26)
Liability derivatives, gross asset	25	7
Derivative liability offsets under cash collateral pledged	(23)	7
Derivative liabilities net amount on balance sheet	(9)	(12)
Derivative liabilities received under securities collateral	0	3
Derivative liability, fair value, amount offset against collateral	$ (9)	$ (9)