Guarantees and Commitments Guarantees and Commitments (Contractual Financing Commitments) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Guarantees and Commitments [Abstract]
Unused revolving credit line commitments and other	$ 2,341	$ 1,995
Commitment to fund remaining unused balances on home equity lines of credit	318	356
Commitments to provide capital to investees	283	206
Warehouse and construction lending commitments	144	164
Mortgage loan origination commitments	$ 95	$ 0