Goodwill and other intangible assets - Key Economic Assumptions (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Weighted-average prepayment speed assumption (“CPR”)	10.19%	9.80%
Impact on fair value of 10% adverse change	$ (297)	$ (337)
Impact on fair value of 20% adverse change	$ (571)	$ (652)
Weighted-average option adjusted spread	9.09%	9.43%
Impact on fair value of 100 basis points adverse change	$ (265)	$ (300)
Impact on fair value of 200 basis points adverse change	$ (510)	$ (578)