Goodwill and Other Intangible Assets - Changes in the Carrying Amounts of Other Intangible Assets (Detail) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017	Dec. 31, 2017		Dec. 31, 2016
Finite-Lived Intangible Assets, Accumulated Amortization	[1]	$ (41)			$ (42)
Intangible Assets, Net (Excluding Goodwill)		2,036	[1]	$ 1,689	1,791	[1]	$ 1,657
Amortization	[2]	(11)		(10)
Origination of Mortgage Servicing Rights (MSRs)		156		169
Servicing Assets at Fair Value, Purchased		75
Due to changes in inputs or assumptions	[3]	146		(16)
Servicing Asset at Fair Value, Other Changes in Fair Value	[4]	(120)		(109)
Servicing Asset at Fair Value, Disposals		1		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]	2,077			1,833
Mortgage Servicing Rights, Fair Value [Member]
Servicing Asset at Fair Value, Amount		1,959		1,608	1,710		1,572
Amortization	[2]	0		0
Origination of Mortgage Servicing Rights (MSRs)		149		162
Servicing Assets at Fair Value, Purchased		75
Due to changes in inputs or assumptions	[3]	146		(16)
Servicing Asset at Fair Value, Other Changes in Fair Value	[4]	(120)		(109)
Servicing Asset at Fair Value, Disposals		1		1
Intangible Assets, Written off Related to Sale of Business Unit	[5]			0
Other Intangible Assets [Member]
Finite-Lived Intangible Assets, Gross	[1]	20			32
Finite-Lived Intangible Assets, Accumulated Amortization	[1]	(18)			(28)
Finite-Lived Intangible Assets, Net	[1]	2			4
Intangible Assets, Net (Excluding Goodwill)		77		81	81		$ 85
Amortization	[2]	(11)		(10)
Origination of Mortgage Servicing Rights (MSRs)		7		7
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]	86			79
Finite-Lived Intangible Assets, Accumulated Amortization	[1]	(23)			(14)
Finite-Lived Intangible Assets, Net	[1]	63			65
Fair Value, Measurements, Recurring [Member]
Servicing Asset at Fair Value, Amount		$ 1,959			$ 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 non-qualified 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.