Loans - Schedule of Non-Covered and Covered Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	$ 14,451,244	$ 14,327,428
Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	10,336,672	10,198,855
Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	6,230,628	6,073,511
Commercial Loans | Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	3,374,382	3,444,578
Commercial Loans | Energy-Related
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	731,662	680,766
Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,208,391	1,195,319
Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,906,181	2,933,254
Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,091,514	2,066,167
Consumer and Other Loans | Indirect Automobile
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	213,179	246,298
Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	601,488	620,789
Legacy Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	11,528,697	11,190,520
Legacy Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	8,427,154	8,133,341
Legacy Loans | Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	4,771,690	4,504,062
Legacy Loans | Commercial Loans | Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,926,686	2,952,102
Legacy Loans | Commercial Loans | Energy-Related
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	728,778	677,177
Legacy Loans | Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	730,621	694,023
Legacy Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,370,922	2,363,156
Legacy Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,625,812	1,575,643
Legacy Loans | Consumer and Other Loans | Indirect Automobile
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	213,141	246,214
Legacy Loans | Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	531,969	541,299
Acquired Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,922,547	3,136,908
Acquired Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,909,518	2,065,514
Acquired Loans | Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,458,938	1,569,449
Acquired Loans | Commercial Loans | Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	447,696	492,476
Acquired Loans | Commercial Loans | Energy-Related
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,884	3,589
Acquired Loans | Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	477,770	501,296
Acquired Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	535,259	570,098
Acquired Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	465,702	490,524
Acquired Loans | Consumer and Other Loans | Indirect Automobile
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	38	84
Acquired Loans | Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	$ 69,519	$ 79,490