Securities Available for Sale (Detail) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 31,347	$ 30,729
Unrealized Gains		323	335
Unrealized Losses		226	392
Available-for-sale Securities	[1]	31,444	30,672
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		4,300	5,486
Unrealized Gains		9	5
Unrealized Losses		48	86
Available-for-sale Securities		4,261	5,405
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		266	310
Unrealized Gains		5	5
Unrealized Losses		1	2
Available-for-sale Securities		270	313
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		558	279
Unrealized Gains		9	5
Unrealized Losses		4	5
Available-for-sale Securities		563	279
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		24,860	23,642
Unrealized Gains		287	313
Unrealized Losses		167	293
Available-for-sale Securities		24,980	23,662
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		59	71
Unrealized Gains		4	3
Unrealized Losses		1	0
Available-for-sale Securities		62	74
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		747	257
Unrealized Gains		6	0
Unrealized Losses		3	5
Available-for-sale Securities		750	252
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		6	8
Unrealized Gains		2	2
Unrealized Losses		0	0
Available-for-sale Securities		8	10
Other Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		33	34
Unrealized Gains		0	1
Unrealized Losses		0	0
Available-for-sale Securities		33	35
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[2]	518	642
Unrealized Gains	[2]	1	1
Unrealized Losses	[2]	2	1
Available-for-sale Securities	[2]	$ 517	$ 642

[1] Includes securities AFS pledged as collateral where counterparties have the right to sell or repledge the collateral of $280 million and $0 million at September 30, 2017 and December 31, 2016, respectively.

[2] At September 30, 2017, the fair value of other equity securities was comprised of the following: $68 million of FHLB of Atlanta stock, $403 million of Federal Reserve Bank of Atlanta stock, $41 million of mutual fund investments, and $5 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.