Securities Available for Sale (Detail) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		$ 31,902	$ 30,913	[1]
Unrealized Gains		117	250	[1]
Unrealized Losses		1,035	216	[1]
Available-for-sale Securities	[2],[3]	30,984	30,947	[1]
Debt Securities, Held-to-maturity, Fair Value		0	0
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		4,275	4,361
Unrealized Gains		0	2
Unrealized Losses		142	32
Available-for-sale Securities		4,133	4,331
US Government Agencies Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		224	257
Unrealized Gains		2	3
Unrealized Losses		3	1
Available-for-sale Securities		223	259
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		621	618
Unrealized Gains		3	7
Unrealized Losses		22	8
Available-for-sale Securities		602	617
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost			6
Unrealized Gains			2
Unrealized Losses			0
Available-for-sale Securities			8
Other Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		14	17
Unrealized Gains		0	0
Unrealized Losses		0	0
Available-for-sale Securities		14	17
Commercial Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		2,713	2,121
Unrealized Gains		1	3
Unrealized Losses		112	38
Available-for-sale Securities		2,602	2,086
Commercial Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		943	862
Unrealized Gains		0	7
Unrealized Losses		38	3
Available-for-sale Securities		905	866
Residential Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		23,112	22,616
Unrealized Gains		111	222
Unrealized Losses		718	134
Available-for-sale Securities		$ 22,505	22,704
Residential Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost			55
Unrealized Gains			4
Unrealized Losses			0
Available-for-sale Securities			$ 59

[1] Beginning January 1, 2018, the Company reclassified equity securities previously presented in Securities available for sale to Other assets on the Consolidated Balance Sheets. Reclassifications have been made to previously reported amounts for comparability. See Note 9, "Other Assets," for additional information.

[2] Beginning January 1, 2018, the Company reclassified equity securities previously presented in Securities available for sale to Other assets. Reclassifications have been made to previously reported amounts for comparability.

[3] Includes securities AFS pledged as collateral where counterparties have the right to sell or repledge the collateral of $164 million and $223 million at September 30, 2018 and December 31, 2017, respectively.