Securities Available for Sale (Detail) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 28,804	$ 27,568
Unrealized Gains		875	424
Unrealized Losses		7	167
Available-for-sale Securities		29,672	27,825
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		4,850	3,460
Unrealized Gains		135	3
Unrealized Losses		2	14
Available-for-sale Securities		4,983	3,449
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		324	402
Unrealized Gains		10	10
Unrealized Losses		0	1
Available-for-sale Securities		334	411
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		250	156
Unrealized Gains		11	8
Unrealized Losses		0	0
Available-for-sale Securities		261	164
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		22,606	22,877
Unrealized Gains		714	397
Unrealized Losses		4	150
Available-for-sale Securities		23,316	23,124
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		75	92
Unrealized Gains		1	2
Unrealized Losses		0	0
Available-for-sale Securities		76	94
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		9	11
Unrealized Gains		2	2
Unrealized Losses		0	1
Available-for-sale Securities		11	12
Other Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		35	37
Unrealized Gains		1	1
Unrealized Losses		0	0
Available-for-sale Securities		36	38
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[1]	655	533
Unrealized Gains	[1]	1	1
Unrealized Losses	[1]	1	1
Available-for-sale Securities	[1]	$ 655	$ 533

[1] At September 30, 2016, the fair value of other equity securities was comprised of the following: $143 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, $104 million of mutual fund investments, and $6 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.