Loans - Schedule of Non-Covered Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	$ 15,132,202	$ 15,064,971
Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	10,997,075	10,906,581
Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	6,977,874	6,802,266
Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	3,455,578	3,543,122
Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	563,623	561,193
Residential Mortgage
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,296,358	1,267,400
Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	2,838,769	2,890,990
Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	2,146,796	2,155,926
Consumer and Other Loans | Consumer - Indirect automobile
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	110,200	131,052
Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	581,773	604,012
Legacy Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	12,923,444	12,694,924
Legacy Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	9,581,229	9,377,399
Legacy Loans | Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	5,878,509	5,623,314
Legacy Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	3,140,205	3,194,796
Legacy Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	562,515	559,289
Legacy Loans | Residential Mortgage
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	901,859	854,216
Legacy Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	2,440,356	2,463,309
Legacy Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,797,123	1,783,421
Legacy Loans | Consumer and Other Loans | Consumer - Indirect automobile
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	110,174	131,048
Legacy Loans | Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	533,059	548,840
Acquired Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	2,208,758	2,370,047
Acquired Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,415,846	1,529,182
Acquired Loans | Commercial Loans | Real Estate
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,099,365	1,178,952
Acquired Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	315,373	348,326
Acquired Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,108	1,904
Acquired Loans | Residential Mortgage
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	394,499	413,184
Acquired Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	398,413	427,681
Acquired Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	349,673	372,505
Acquired Loans | Consumer and Other Loans | Consumer - Indirect automobile
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	26	4
Acquired Loans | Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	$ 48,714	$ 55,172