Variable interest entities - Loans Sold to Third-Party Sponsored Securitization Entities (Details) - Nonconsolidated entities - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Summary of loan sale activities [Abstract]
Carrying value of loans sold	$ 11,394	$ 12,396	$ 34,193	$ 38,919
Proceeds received from loan sales as cash	139	77	238	166
Proceeds from loans sales as securities	11,170	12,250	33,758	38,446
Total proceeds received from loan sales	11,309	12,327	33,996	38,612
Gains on loan sales	61	$ 86	238	$ 205
Loans repurchased	11,300		11,300		$ 12,400
Real estate acquired through foreclosure	$ 327		$ 327		$ 464