Derivatives - Summary of gross notional amount of derivatives (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Interest rate lock commitments expected to close | Minimum
Derivatives
Loan commitment period	30 days
Interest rate lock commitments expected to close | Maximum
Derivatives
Loan commitment period	90 days
Not designated as hedging instruments | Interest rate lock commitments expected to close
Notional amount of derivatives
Notional	$ 192	$ 862
Not designated as hedging instruments | Forward delivery commitments
Notional amount of derivatives
Notional	614	2,104
Not designated as hedging instruments | MSR-related agreements
Notional amount of derivatives
Notional	0	260
Not designated as hedging instruments | Option contracts
Notional amount of derivatives
Notional	$ 0	$ 120