Securities Available for Sale (Detail) - USD ($) $ in Millions		6 Months Ended	12 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 31,627	$ 30,913	[1]
Unrealized Gains		140	250	[1]
Unrealized Losses		825	216	[1]
Available-for-sale Securities	[2],[3]	30,942	30,947	[1]
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		4,239	4,361
Unrealized Gains		0	2
Unrealized Losses		113	32
Available-for-sale Securities		4,126	4,331
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		244	257
Unrealized Gains		1	3
Unrealized Losses		2	1
Available-for-sale Securities		243	259
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		632	618
Unrealized Gains		4	7
Unrealized Losses		21	8
Available-for-sale Securities		615	617
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			6
Unrealized Gains			2
Unrealized Losses			0
Available-for-sale Securities			8
Other Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		15	17
Unrealized Gains		0	0
Unrealized Losses		0	0
Available-for-sale Securities		15	17
Commercial Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		2,664	2,121
Unrealized Gains		1	3
Unrealized Losses		97	38
Available-for-sale Securities		2,568	2,086
Commercial Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		950	862
Unrealized Gains		0	7
Unrealized Losses		34	3
Available-for-sale Securities		916	866
Residential Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		22,883	22,616
Unrealized Gains		134	222
Unrealized Losses		558	134
Available-for-sale Securities		$ 22,459	22,704
Residential Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [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 $184 million and $223 million at June 30, 2018 and December 31, 2017, respectively.