Bank segment - Repurchase Agreements (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Offsetting Liabilities [Line Items]
Gross amount of recognized liabilities	$ 50	$ 141
Gross amount offset in the Balance Sheet	0	0
Securities sold under agreements to repurchase	50	141
Securities sold under agreements to repurchase collateral, financial instruments	97	165
Securities sold under agreements to repurchase, cash collateral pledged	0	0
Commercial account holders
Offsetting Liabilities [Line Items]
Securities sold under agreements to repurchase	50	141
Securities sold under agreements to repurchase collateral, financial instruments	97	165
Securities sold under agreements to repurchase, cash collateral pledged	$ 0	$ 0