Securities Available for Sale (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 31,385	$ 30,729
Unrealized Gains		250	335
Unrealized Losses		219	392
Available-for-sale Securities	[1]	31,416	30,672
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		4,361	5,486
Unrealized Gains		2	5
Unrealized Losses		32	86
Available-for-sale Securities		4,331	5,405
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		257	310
Unrealized Gains		3	5
Unrealized Losses		1	2
Available-for-sale Securities		259	313
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		618	279
Unrealized Gains		7	5
Unrealized Losses		8	5
Available-for-sale Securities		617	279
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		17	34
Unrealized Gains		0	1
Unrealized Losses		0	0
Available-for-sale Securities		17	35
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	[2]	472	642
Unrealized Gains	[2]	0	1
Unrealized Losses	[2]	3	1
Available-for-sale Securities	[2]	469	642
Commercial Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		2,121	1,263
Unrealized Gains		3	2
Unrealized Losses		38	39
Available-for-sale Securities		2,086	1,226
Commercial Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		862	257
Unrealized Gains		7	0
Unrealized Losses		3	5
Available-for-sale Securities		866	252
Residential Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		22,616	22,379
Unrealized Gains		222	311
Unrealized Losses		134	254
Available-for-sale Securities		22,704	22,436
Residential Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		55	71
Unrealized Gains		4	3
Unrealized Losses		0	0
Available-for-sale Securities		$ 59	$ 74

[1] Includes securities AFS pledged as collateral where counterparties have the right to sell or repledge the collateral of $223 million and $0 million at December 31, 2017 and December 31, 2016, respectively.

[2] At December 31, 2017, the fair value of other equity securities was comprised of the following: $15 million of FHLB of Atlanta stock, $403 million of Federal Reserve Bank of Atlanta stock, $49 million of mutual fund investments, and $2 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.