Goodwill and Other Intangible Assets - Changes in the Carrying Amounts of Other Intangible Assets (Detail) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2018		Sep. 30, 2017	Dec. 31, 2017		Dec. 31, 2016
Finite-Lived Intangible Assets, Accumulated Amortization	[1]	$ (42)			$ (42)
Intangible Assets, Net (Excluding Goodwill)		2,140	[1]	$ 1,706	1,791	[1]	$ 1,657
Amortization	[2]	(13)		(16)
Origination of Mortgage Servicing Rights (MSRs)		260		262
Servicing Assets at Fair Value, Purchased		89
Due to changes in inputs or assumptions	[3]	198		(27)
Servicing Asset at Fair Value, Other Changes in Fair Value	[4]	(183)		(168)
Servicing Asset at Fair Value, Disposals		2		1
Intangible Assets, Written off Related to Sale of Business Unit	[5]			(1)
Indefinite-lived Intangible Assets (Excluding Goodwill)		12			12
Intangible Assets, Gross (Excluding Goodwill)	[1]	2,182			1,833
Mortgage Servicing Rights, Fair Value [Member]
Servicing Asset at Fair Value, Amount		2,062		1,628	1,710		1,572
Amortization	[2]	0		0
Origination of Mortgage Servicing Rights (MSRs)		250		252
Servicing Assets at Fair Value, Purchased		89
Due to changes in inputs or assumptions	[3]	198		(27)
Servicing Asset at Fair Value, Other Changes in Fair Value	[4]	(183)		(168)
Servicing Asset at Fair Value, Disposals		2		1
Intangible Assets, Written off Related to Sale of Business Unit	[5]			0
Other Intangible Assets [Member]
Finite-Lived Intangible Assets, Gross	[1]	19			32
Finite-Lived Intangible Assets, Accumulated Amortization	[1]	(17)			(28)
Finite-Lived Intangible Assets, Net	[1]	2			4
Intangible Assets, Net (Excluding Goodwill)		78		78	81		$ 85
Amortization	[2]	(13)		(16)
Origination of Mortgage Servicing Rights (MSRs)		10		10
Servicing Assets at Fair Value, Purchased		0
Due to changes in inputs or assumptions	[3]	0		0
Servicing Asset at Fair Value, Other Changes in Fair Value	[4]	0		0
Servicing Asset at Fair Value, Disposals		0		0
Intangible Assets, Written off Related to Sale of Business Unit	[5]			$ (1)
Commercial Mortgage Servicing Rights [Member]
Finite-Lived Intangible Assets, Gross	[1]	89			79
Finite-Lived Intangible Assets, Accumulated Amortization	[1]	(25)			(14)
Finite-Lived Intangible Assets, Net	[1]	64			65
Fair Value, Measurements, Recurring [Member]
Servicing Asset at Fair Value, Amount		$ 2,062			$ 1,710

[1] Excludes other intangible assets that are indefinite-lived, carried at fair value, or fully amortized.

[2] Does not include expense associated with community development investments. See Note 10, "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.

[5] Represents measurement period adjustment on other intangible assets acquired previously in the Pillar acquisition.