Loans - Schedule of Non-Covered and Covered Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	$ 14,924,499	$ 14,327,428
Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	10,743,853	10,198,855
Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	6,681,215	6,073,511
Commercial Loans | Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	3,462,997	3,444,578
Commercial Loans | Energy-Related
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	599,641	680,766
Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,270,530	1,195,319
Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,910,116	2,933,254
Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,151,130	2,066,167
Consumer and Other Loans | Indirect Automobile
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	153,913	246,298
Consumer and Other Loans | Consumer - Other
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	605,073	620,789
Legacy Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	12,413,370	11,190,520
Legacy Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	9,119,234	8,133,341
Legacy Loans | Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	5,419,483	4,504,062
Legacy Loans | Commercial Loans | Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	3,101,472	2,952,102
Legacy Loans | Commercial Loans | Energy-Related
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	598,279	677,177
Legacy Loans | Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	840,082	694,023
Legacy Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,454,054	2,363,156
Legacy Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,755,295	1,575,643
Legacy Loans | Consumer and Other Loans | Indirect Automobile
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	153,904	246,214
Legacy Loans | Consumer and Other Loans | Consumer - Other
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	544,855	541,299
Acquired Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	2,511,129	3,136,908
Acquired Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,624,619	2,065,514
Acquired Loans | Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,261,732	1,569,449
Acquired Loans | Commercial Loans | Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	361,525	492,476
Acquired Loans | Commercial Loans | Energy-Related
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	1,362	3,589
Acquired Loans | Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	430,448	501,296
Acquired Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	456,062	570,098
Acquired Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	395,835	490,524
Acquired Loans | Consumer and Other Loans | Indirect Automobile
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	9	84
Acquired Loans | Consumer and Other Loans | Consumer - Other
Accounts, Notes, Loans and Financing Receivable
Non-covered loans, net of unearned income	$ 60,218	$ 79,490