Loans - Schedule of Non-Covered and Covered Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	$ 14,722,561	$ 14,327,428
Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	10,569,844	10,198,855
Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	6,472,001	6,073,511
Commercial Loans | Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	3,435,809	3,444,578
Commercial Loans | Energy-Related
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	662,034	680,766
Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,249,062	1,195,319
Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,903,655	2,933,254
Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,129,812	2,066,167
Consumer and Other Loans | Indirect Automobile
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	182,223	246,298
Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	591,620	620,789
Legacy Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	11,984,849	11,190,520
Legacy Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	8,784,789	8,133,341
Legacy Loans | Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	5,097,689	4,504,062
Legacy Loans | Commercial Loans | Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	3,027,590	2,952,102
Legacy Loans | Commercial Loans | Energy-Related
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	659,510	677,177
Legacy Loans | Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	794,701	694,023
Legacy Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,405,359	2,363,156
Legacy Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,695,113	1,575,643
Legacy Loans | Consumer and Other Loans | Indirect Automobile
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	182,199	246,214
Legacy Loans | Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	528,047	541,299
Acquired Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,737,712	3,136,908
Acquired Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,785,055	2,065,514
Acquired Loans | Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,374,312	1,569,449
Acquired Loans | Commercial Loans | Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	408,219	492,476
Acquired Loans | Commercial Loans | Energy-Related
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,524	3,589
Acquired Loans | Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	454,361	501,296
Acquired Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	498,296	570,098
Acquired Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	434,699	490,524
Acquired Loans | Consumer and Other Loans | Indirect Automobile
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	24	84
Acquired Loans | Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	$ 63,573	$ 79,490