Securities Available for Sale (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 27,487	$ 27,568
Unrealized Gains		718	424
Unrealized Losses		17	167
Available-for-sale Securities		28,188	27,825
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		3,694	3,460
Unrealized Gains		103	3
Unrealized Losses		0	14
Available-for-sale Securities		3,797	3,449
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		377	402
Unrealized Gains		12	10
Unrealized Losses		0	1
Available-for-sale Securities		389	411
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		149	156
Unrealized Gains		9	8
Unrealized Losses		0	0
Available-for-sale Securities		158	164
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		22,615	22,877
Unrealized Gains		589	397
Unrealized Losses		14	150
Available-for-sale Securities		23,190	23,124
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		88	92
Unrealized Gains		1	2
Unrealized Losses		1	0
Available-for-sale Securities		88	94
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		10	11
Unrealized Gains		2	2
Unrealized Losses		1	1
Available-for-sale Securities		11	12
Other Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		36	37
Unrealized Gains		1	1
Unrealized Losses		0	0
Available-for-sale Securities		37	38
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[1]	518	533
Unrealized Gains	[1]	1	1
Unrealized Losses	[1]	1	1
Available-for-sale Securities	[1]	$ 518	$ 533

[1] At March 31, 2016, the fair value of other equity securities was comprised of the following: $64 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, $47 million of mutual fund investments, and $5 million of other.At December 31, 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, $93 million of mutual fund investments, and $6 million of other.