Consolidations and Transfers of Financial Assets Transfers of Financial Assets (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Transfers of Financial Assets [Line Items]
Portfolio securitizations: unpaid principal balance	$ 64,900	$ 68,100	$ 180,800	$ 194,500
Unconsolidated VIEs [Member]
Transfers of Financial Assets [Line Items]
Retained interests: principal and interest received	114	$ 294	468	$ 854
Unconsolidated VIEs [Member] | Single Class MBS, REMIC & Megas [Member]
Transfers of Financial Assets [Line Items]
Retained interests: unpaid principal balance	1,500		1,500		$ 3,900
Retained interests: fair value	$ 2,200		$ 2,200		$ 4,700