Bank segment - Repurchase Agreements (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Offsetting Liabilities [Line Items]
Gross amount of recognized liabilities	$ 173	$ 229
Securities sold under agreements to repurchase	173	229
Securities sold under agreements to repurchase collateral, financial instruments	216	261
Securities sold under agreements to repurchase, cash collateral pledged	0	0
Financial Institution
Offsetting Liabilities [Line Items]
Securities sold under agreements to repurchase	50	50
Securities sold under agreements to repurchase collateral, financial instruments	57	56
Securities sold under agreements to repurchase, cash collateral pledged	0	0
Government Entities
Offsetting Liabilities [Line Items]
Securities sold under agreements to repurchase	27	56
Securities sold under agreements to repurchase collateral, financial instruments	42	61
Securities sold under agreements to repurchase, cash collateral pledged	0	0
Commercial account holders
Offsetting Liabilities [Line Items]
Securities sold under agreements to repurchase	96	123
Securities sold under agreements to repurchase collateral, financial instruments	117	144
Securities sold under agreements to repurchase, cash collateral pledged	$ 0	$ 0