Goodwill and other intangible assets - Key Economic Assumptions (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Weighted-average prepayment speed assumption (“CPR”)	9.01%	9.80%
Impact on fair value of 10% adverse change	$ (327)	$ (337)
Impact on fair value of 20% adverse change	$ (633)	$ (652)
Weighted-average option adjusted spread	9.38%	9.43%
Impact on fair value of 100 basis points adverse change	$ (304)	$ (300)
Impact on fair value of 200 basis points adverse change	$ (586)	$ (578)