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
Asset derivatives gross amount	$ 115	$ 103
Asset derivatives, Gross liability	0	(2)
Total asset derivatives, Fair value, net	115	101
Liability derivatives
Liability derivatives gross amount	(346)	(329)
Liability derivatives, Gross asset	3	0
Derivative liabilities net amount on balance sheet	(343)	(329)
OTC derivatives
Asset derivatives
Asset derivatives gross amount	4	14
Asset derivatives, Gross liability	(3)	(2)
Derivative asset offsets under cash collateral received	0	(4)
Total asset derivatives, Fair value, net	1	8
Derivative assets pledged under securities collateral	0	(1)
Asset derivatives, net amount	1	7
Liability derivatives
Liability derivatives gross amount	(10)	(5)
Liability derivatives, Gross asset	3	2
Derivative liability offsets under cash collateral pledged	(2)	0
Derivative liabilities net amount on balance sheet	(9)	(3)
Derivative liabilities received under securities collateral	3	4
Liability derivatives, net amount	$ (6)	$ 1