Bank Premises and Equipment (Bank Premises and Equipment - Additional Information (Detail)) $ in Millions		3 Months Ended			12 Months Ended
		Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Property, Plant and Equipment
Depreciation and amortization expense					$ 242	$ 256	$ 254
Rental income from leased premises					16	18	17
Gain on Branch Consolidation and Sales Plan					19	0	0
Land and improvements held for sale					29	55
Buildings held for sale					9	20
Equipment Held For Sale					1	3
Lease Term		15
Land and improvements	[1]				663	685
Buildings	[1]				1,672	1,755
Equipment					$ 1,761	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
Deferred Gain on Transaction		10
Gain on Sale Leaseback Transaction		$ 11
Disposal group classified as held for sale
Property, Plant and Equipment
Operating Branch Locations					64
Parcels Of Undeveloped Land					35
Disposal group classified as held for sale | September 13 2016 Announcement
Property, Plant and Equipment
Parcels Of Undeveloped Land		5
Branch Banking
Property, Plant and Equipment
Bank Premises Impairment					$ 32	109	20
Branches and undeveloped parcels of land
Property, Plant and Equipment
Land and improvements					92	102
Branches and undeveloped parcels of land | September 13 2016 Announcement
Property, Plant and Equipment
Operating Branch Locations		44
St. Louis MSA to Great Southern Bank
Property, Plant and Equipment
Gain on Branch Consolidation and Sales Plan				$ 8
Branch Consolidation And Sales Plan | Disposal group classified as held for sale | Bank Premises And Equipment | September 13 2016 Announcement
Property, Plant and Equipment
Land and improvements held for sale					29
Buildings held for sale					9
Equipment Held For Sale					1
Land and improvements					39
Buildings					16
Equipment					1
Pittsburgh MSA To First National Bank of Pennsylvania
Property, Plant and Equipment
Gain on Branch Consolidation and Sales Plan			$ 11
Cancelable And Noncancelable Lease Obligations
Property, Plant and Equipment
Occupancy Costs					$ 100	$ 110	$ 100

[1] (a) At December 31, 2016 and 2015 , land and improvements, buildings and construction in progress included $ 92 and $ 102 , respectively, associated with parcels of undeveloped land intended for future branch expansion.