Intangible Assets - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2012	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Outstanding principal balances at period end of residential mortgage loans serviced for others	$ 20,266		$ 20,266			$ 23,101
Outstanding principal balances at period end of residential mortgage loans owned and serviced for others	38,786		38,786			$ 40,889
Purchased from HSBC Finance account of credit card					$ 746
Credit card receivables not being sold to Capital One at a fair value					108
Fair value of Intangible asset related to account relationships purchased include in branch sale					43
Fair value of Intangible asset related to account relationships purchased retained					$ 65
Residential mortgage banking revenue [Member]
Finite-Lived Intangible Assets [Line Items]
Servicing fees collected	$ 13	$ 17	$ 43	$ 52
Purchased credit card relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Useful life					10 years