Fair Value Measurements - Unrealized gains (losses) related to assets and liabilities classified with Level Three valuation hierarchy (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Gain on loans held for sale, net
Unrealized gains (losses) related to assets and liabilities included in Condensed Consolidated Balance Sheets
Unrealized Gain or Loss for Level 3 Fair Value instruments	$ 2	$ 4	$ 4	$ 7
Loan servicing income, net
Unrealized gains (losses) related to assets and liabilities included in Condensed Consolidated Balance Sheets
Unrealized Gain or Loss for Level 3 Fair Value instruments	$ (2)	$ (4)	$ (1)	$ (6)

Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

Name:	us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration