Goodwill and Other Intangible Assets - Changes in the Carrying Amounts of Other Intangible Assets (Detail) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2015		Sep. 30, 2014		Jun. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Principal Amount Outstanding on Loans Managed and Securitized or Asset-backed Financing Arrangement	[1]	$ 256,461					$ 248,646
Servicing Asset at Fair Value, Amount		1,262					1,206
Intangible Assets, Net (Excluding Goodwill)		1,282		$ 1,320			1,219	$ 1,334
Amortization	[2]	(6)		(10)
Origination of Mortgage Servicing Rights (MSRs)		198		137
Servicing Assets at Fair Value, Purchased		109		109
Due to changes in inputs or assumptions	[3]	(74)		(117)
Servicing Asset at Fair Value, Other Changes in Fair Value	[4]	(161)		(123)
Servicing Asset at Fair Value, Disposals		(3)		(1)
Intangible Assets, Written off Related to Sale of Business Unit				(9)
indirect auto loan servicing rights [Member]
Servicing Asset						$ 13
Servicing Asset at Fair Value, Amount		11
Mortgage Servicing Rights, Fair Value [Member]
Principal Amount Outstanding on Loans Managed and Securitized or Asset-backed Financing Arrangement		149,200					142,100
Servicing Asset at Fair Value, Amount		1,262		1,305			1,206	1,300
Amortization		0		0
Origination of Mortgage Servicing Rights (MSRs)		185		137
Servicing Assets at Fair Value, Purchased		109		109
Due to changes in inputs or assumptions		(74)	[2]	(117)	[3]
Servicing Asset at Fair Value, Other Changes in Fair Value	[4]	(161)		(123)
Servicing Asset at Fair Value, Disposals		(3)		(1)
Intangible Assets, Written off Related to Sale of Business Unit				0
Principal Amount Outstanding of Loans Serviced For Third Parties		122,000					115,500
Unpaid Principal Balance of Outstanding Underlying MSRs Purchased		10,300		9,000
Principal Amount Sold on Loans Serviced for Third Parties		590		612
Other Intangible Assets [Member]
Intangible Assets, Net (Excluding Goodwill)		20		15			$ 13	$ 34
Amortization	[2]	(6)		(10)
Origination of Mortgage Servicing Rights (MSRs)		13		0
Servicing Assets at Fair Value, Purchased		0		0
Due to changes in inputs or assumptions		0		0
Servicing Asset at Fair Value, Other Changes in Fair Value		0		0
Servicing Asset at Fair Value, Disposals		$ 0		0
Intangible Assets, Written off Related to Sale of Business Unit				$ (9)

[1] Excludes $2.0 billion and $3.2 billion of LHFS at September 30, 2015 and December 31, 2014, respectively.

[2] Does not include amortization of tax credits for non-qualified community development investments. See Note 8, "Certain Transfers of Financial Assets and Variable Interest Entities," for additional information.

[3] Primarily reflects changes in option adjusted spreads and prepayment speed assumptions, due to changes in interest rates.

[4] Represents changes due to the collection of expected cash flows, net of accretion, due to the passage of time.