Securities Available for Sale (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 31,181	$ 30,729
Unrealized Gains		316	335
Unrealized Losses		370	392
Available-for-sale Securities	[1]	31,127	30,672
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		5,488	5,486
Unrealized Gains		5	5
Unrealized Losses		71	86
Available-for-sale Securities		5,422	5,405
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		297	310
Unrealized Gains		6	5
Unrealized Losses		1	2
Available-for-sale Securities		302	313
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		319	279
Unrealized Gains		7	5
Unrealized Losses		4	5
Available-for-sale Securities		322	279
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		23,791	23,642
Unrealized Gains		291	313
Unrealized Losses		287	293
Available-for-sale Securities		23,795	23,662
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		68	71
Unrealized Gains		3	3
Unrealized Losses		0	0
Available-for-sale Securities		71	74
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		539	257
Unrealized Gains		1	0
Unrealized Losses		6	5
Available-for-sale Securities		534	252
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		7	8
Unrealized Gains		2	2
Unrealized Losses		0	0
Available-for-sale Securities		9	10
Other Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		34	34
Unrealized Gains		0	1
Unrealized Losses		0	0
Available-for-sale Securities		34	35
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[2]	638	642
Unrealized Gains	[2]	1	1
Unrealized Losses	[2]	1	1
Available-for-sale Securities	[2]	$ 638	$ 642

[1] Includes securities AFS pledged as collateral where counterparties have the right to sell or repledge the collateral of $100 million and $0 million at March 31, 2017 and December 31, 2016, respectively.

[2] At March 31, 2017, the fair value of other equity securities was comprised of the following: $111 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, $119 million of mutual fund investments, and $6 million of other.At December 31, 2016, the fair value of other equity securities was comprised of the following: $132 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, $102 million of mutual fund investments, and $6 million of other.