Derivatives - Summary of gross notional amount of derivatives (Details) - Not designated as hedging instruments - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Interest rate lock commitments
Notional amount of derivatives
Notional	$ 51	$ 139
Forward delivery commitments
Notional amount of derivatives
Notional	$ 126	$ 614

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant