Commitments and Contingencies - Schedule of Commitments and Contingencies (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Letters of credit	$ 2,466	$ 2,786
Carrying amount of the liability for letters of credit	21	27
Investments in affordable housing and historic building rehabilitation projects:
Carrying amount	1,948	1,719
Amount of future funding commitments included in carrying amount	928	738
Lending exposure	561	495
Tax credits subject to recapture	471	413
Private equity investments	471	417
Future funding commitments to private equity investments	$ 143	$ 199