Loan Servicing - Rollforward of Residential MSRs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Servicing Asset at Fair Value, Amount [Roll Forward]
Carrying value, beginning of year	$ 1,052
Change in fair value due to changes in valuation inputs or assumptions:
Carrying value, end of year	1,056	$ 1,052
Residential MSRs
Servicing Asset at Fair Value, Amount [Roll Forward]
Carrying value, beginning of year	915	880	$ 844
Additions	123	146	156
Change in fair value due to changes in valuation inputs or assumptions:
Prepayment speeds	(42)	13	91
Weighted average OAS	46	10	(52)
Servicing costs	9	2	(25)
Realization of expected net servicing cash flows, passage of time and other	(137)	(136)	(134)
Carrying value, end of year	914	915	880
Gains (losses) on derivative financial instruments used to mitigate the income statement effect of changes in fair value	$ 0	$ 32	$ 32