Netting Arrangements Narratives (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Fair value of non-cash collateral we pledged	$ 747,000,000	$ 1,300,000,000
Fair value of non-cash collateral received	44,700,000,000	51,200,000,000
Fair value of non-cash collateral received that could be sold or repledged	42,500,000,000	45,500,000,000
Fair value of underlying collateral that was sold or repledged	0	0
Derivative assets not subject to an enforceable master netting arrangement	21,000,000	33,000,000
Derivative liabilities not subject to an enforceable master netting arrangement	1,000,000	2,000,000
Cash and cash equivalents [Member]
Derivative [Line Items]
Securities purchased under agreements to resell	$ 25,200,000,000	$ 20,700,000,000