Loans - Schedule of Non-Covered and Covered Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014		Sep. 30, 2015	Sep. 30, 2014
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	$ 10,996,500		$ 13,863,478
Covered loans receivable	444,544		253,541
Total loans, net of unearned income	11,441,044		14,117,019
Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	7,814,082		10,002,178
Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	4,361,779		5,979,751
Commercial Loans | Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	2,571,695		3,302,971
Commercial Loans | Energy-Related
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	880,608		719,456
Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,080,297		1,189,941
Residential Mortgage Loans | Residential 1-4 Family
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,048,241		1,114,752
Residential Mortgage Loans | Construction / Owner Occupied
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	32,056		75,189
Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	2,546,665		2,924,900
Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,601,105		2,015,687
Consumer and Other Loans | Indirect Automobile
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	397,158		281,649
Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	548,402		627,564
Non-Covered Legacy Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	9,668,714		10,779,258
Non-Covered Legacy Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	7,002,198		7,815,161
Non-Covered Legacy Loans | Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	3,676,811		4,321,723
Non-Covered Legacy Loans | Commercial Loans | Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,452,521		2,779,503
Non-Covered Legacy Loans | Commercial Loans | Energy-Related
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	872,866		713,935
Non-Covered Legacy Loans | Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	527,694		660,543
Non-Covered Legacy Loans | Residential Mortgage Loans | Residential 1-4 Family
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	495,638		596,428
Non-Covered Legacy Loans | Residential Mortgage Loans | Construction / Owner Occupied
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	32,056		64,115
Non-Covered Legacy Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,138,822		2,303,554
Non-Covered Legacy Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,290,976		1,488,796
Non-Covered Legacy Loans | Consumer and Other Loans | Indirect Automobile
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	396,766		281,522
Non-Covered Legacy Loans | Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	451,080		533,236
Non-Covered Acquired Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,327,786		3,084,220
Non-Covered Acquired Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	591,498		2,132,008
Non-Covered Acquired Loans | Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	495,842		1,614,438
Non-Covered Acquired Loans | Commercial Loans | Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	87,914		512,049
Non-Covered Acquired Loans | Commercial Loans | Energy-Related
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	7,742		5,521
Non-Covered Acquired Loans | Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	424,579		409,472
Non-Covered Acquired Loans | Residential Mortgage Loans | Residential 1-4 Family
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	424,579		398,398
Non-Covered Acquired Loans | Residential Mortgage Loans | Construction / Owner Occupied
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	0		11,074
Non-Covered Acquired Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	311,709		542,740
Non-Covered Acquired Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	217,699		449,665
Non-Covered Acquired Loans | Consumer and Other Loans | Indirect Automobile
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	392		127
Non-Covered Acquired Loans | Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	93,618		92,948
Covered Loans
Accounts, Notes, Loans and Financing Receivable
Covered loans receivable	444,544	[1]	253,541	$ 524,189
Assets whose reimbursable loss period ended January 1, 2015	174,700
Covered Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Covered loans receivable	220,386	[1]	55,009
Covered Loans | Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Covered loans receivable	189,126	[1]	43,590	247,156
Covered Loans | Commercial Loans | Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable
Covered loans receivable	31,260	[1]	11,419	34,234
Covered Loans | Commercial Loans | Energy-Related
Accounts, Notes, Loans and Financing Receivable
Covered loans receivable	0	[1]	0
Covered Loans | Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable
Covered loans receivable	128,024	[1]	119,926	130,976
Covered Loans | Residential Mortgage Loans | Residential 1-4 Family
Accounts, Notes, Loans and Financing Receivable
Covered loans receivable	128,024	[1]	119,926
Covered Loans | Residential Mortgage Loans | Construction / Owner Occupied
Accounts, Notes, Loans and Financing Receivable
Covered loans receivable	0	[1]	0
Covered Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Covered loans receivable	96,134	[1]	78,606	$ 111,823
Covered Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Covered loans receivable	92,430	[1]	77,226
Covered Loans | Consumer and Other Loans | Indirect Automobile
Accounts, Notes, Loans and Financing Receivable
Covered loans receivable	0	[1]	0
Covered Loans | Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Covered loans receivable	$ 3,704	[1]	$ 1,380

[1] Included as covered loans at December 31, 2014 is $174.7 million of assets whose reimbursable loss periods ended as of January 1, 2015.