Bank Premises & Equipment (Bank Premises and Equipment - Additional Information) (Detail) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Bank Premises and Equipment
Gain on Branch Consolidation and Sales Plan						$ 19	$ 0
Land and improvements held for sale		$ 34				34		$ 55
Buildings held for sale		10				10		20
Equipment Held For Sale		1				1		3
Land and improvements	[1]	663				663		685
Buildings		1,658				1,658		1,755
Equipment		1,725				1,725		1,696
Proceeds on sale of office complex						$ 31
Sale Leaseback Transaction Other Information						"25% of the office complex"
Sale Leaseback Transaction Net Book Value		10				$ 10
Deferred Gain on Transaction		$ 10				10
Lease Term		15
Gain on Sale Leaseback Transaction		$ 11
Branch Banking
Bank Premises and Equipment
Bank premises impairment		$ 28			$ 2	$ 31	$ 104
Disposal group classified as held for sale | September 13 2016 Announcement
Bank Premises and Equipment
Parcels Of Undeveloped Land		5				5
Branches and undeveloped parcels of land
Bank Premises and Equipment
Land and improvements		$ 94				$ 94		$ 102
Branches and undeveloped parcels of land | Disposal group classified as held for sale | September 13 2016 Announcement
Bank Premises and Equipment
Operating Branch Locations		44				44
St. Louis MSA to Great Southern Bank
Bank Premises and Equipment
Gain on Branch Consolidation and Sales Plan				$ 8
Branch Consolidation and Sales Plan | Disposal group classified as held for sale | September 13 2016 Announcement
Bank Premises and Equipment
Operating Branch Locations		72				72
Parcels Of Undeveloped Land		37				37
Branch Consolidation and Sales Plan | Disposal group classified as held for sale | Bank Premises And Equipment | September 13 2016 Announcement
Bank Premises and Equipment
Land and improvements held for sale		$ 34				$ 34
Buildings held for sale		10				10
Equipment Held For Sale		1				1
Land and improvements		45				45
Buildings		17				17
Equipment		$ 2				$ 2
Pittsburgh MSA To First National Bank of Pennsylvania
Bank Premises and Equipment
Gain on Branch Consolidation and Sales Plan			$ 11

[1] (a ) At September 30, 2016 and December 31, 2015 , land and improvements included $ 94 and $ 102 , respectively, associated w ith parcels of undeveloped land intended for future branch expansion.