Securities Available for Sale (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 31,458	$ 30,913	[1]
Unrealized Gains		160	250	[1]
Unrealized Losses		684	216	[1]
Available-for-sale Securities	[2],[3]	30,934	30,947	[1]
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		4,437	4,361
Unrealized Gains		0	2
Unrealized Losses		97	32
Available-for-sale Securities		4,340	4,331
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		248	257
Unrealized Gains		3	3
Unrealized Losses		2	1
Available-for-sale Securities		249	259
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		644	618
Unrealized Gains		5	7
Unrealized Losses		13	8
Available-for-sale Securities		636	617
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		6	6
Unrealized Gains		1	2
Unrealized Losses		0	0
Available-for-sale Securities		7	8
Other Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		16	17
Unrealized Gains		0	0
Unrealized Losses		0	0
Available-for-sale Securities		16	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		2,320	2,121
Unrealized Gains		1	3
Unrealized Losses		79	38
Available-for-sale Securities		2,242	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		897	862
Unrealized Gains		0	7
Unrealized Losses		23	3
Available-for-sale Securities		874	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		22,837	22,616
Unrealized Gains		146	222
Unrealized Losses		470	134
Available-for-sale Securities		22,513	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		53	55
Unrealized Gains		4	4
Unrealized Losses		0	0
Available-for-sale Securities		$ 57	$ 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] Includes securities AFS pledged as collateral where counterparties have the right to sell or repledge the collateral of $214 million and $223 million at March 31, 2018 and December 31, 2017, respectively.

[3] Text selection found with no content.