Bank segment - Repurchase Agreements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Offsetting Liabilities [Line Items]
Gross amount of recognized liabilities	$ 212	$ 191
Securities sold under agreements to repurchase	212	191
Securities sold under agreements to repurchase collateral, financial instruments	212	191
Financial Institution
Offsetting Liabilities [Line Items]
Securities sold under agreements to repurchase	50	50
Securities sold under agreements to repurchase collateral, financial instruments	50	50
Government Entities
Offsetting Liabilities [Line Items]
Securities sold under agreements to repurchase	56	56
Securities sold under agreements to repurchase collateral, financial instruments	56	56
Commercial account holders
Offsetting Liabilities [Line Items]
Securities sold under agreements to repurchase	106	85
Securities sold under agreements to repurchase collateral, financial instruments	$ 106	$ 85