Securities Available for Sale (Detail) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 26,747	$ 26,257
Unrealized Gains		583	600
Unrealized Losses		60	87
Available-for-sale Securities	[1]	27,270	26,770
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		3,020	1,913
Unrealized Gains		45	9
Unrealized Losses		0	1
Available-for-sale Securities		3,065	1,921
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		408	471
Unrealized Gains		13	15
Unrealized Losses		1	2
Available-for-sale Securities		420	484
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		167	200
Unrealized Gains		7	9
Unrealized Losses		0	0
Available-for-sale Securities		174	209
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		22,452	22,573
Unrealized Gains		511	558
Unrealized Losses		58	83
Available-for-sale Securities		22,905	23,048
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		100	122
Unrealized Gains		2	2
Unrealized Losses		0	1
Available-for-sale Securities		102	123
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		13	19
Unrealized Gains		2	2
Unrealized Losses		0	0
Available-for-sale Securities		15	21
Other Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		36	38
Unrealized Gains		2	3
Unrealized Losses		0	0
Available-for-sale Securities		38	41
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[2]	551	921
Unrealized Gains	[2]	1	2
Unrealized Losses	[2]	1	0
Available-for-sale Securities	[2]	$ 551	$ 923

[1] Includes securities AFS pledged as collateral where counterparties have the right to sell or repledge the collateral of $0 million and $369 million at September 30, 2015 and December 31, 2014, respectively.

[2] At September 30, 2015, the fair value of other equity securities was comprised of the following: $32 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, $111 million of mutual fund investments, and $6 million of other. At December 31, 2014, the fair value of other equity securities was comprised of the following: $376 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, $138 million of mutual fund investments, and $7 million of other.