Loans - Schedule of Non-Covered Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	$ 15,556,016	$ 15,064,971
Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	11,377,652	10,906,581
Commercial Loans | Commercial real estate- construction
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,100,188	802,169
Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	2,205,408	2,234,636
Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	3,836,007	3,765,461
Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	3,684,081	3,543,122
Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	551,968	561,193
Residential Mortgage
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,346,467	1,267,400
Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	2,831,897	2,890,990
Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	2,158,948	2,155,926
Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	92,130	131,052
Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	580,819	604,012
Legacy Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	13,493,410	12,694,924
Legacy Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	10,055,791	9,377,399
Legacy Loans | Commercial Loans | Commercial real estate- construction
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,010,479	740,761
Legacy Loans | Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,815,167	1,784,624
Legacy Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	3,289,284	3,097,929
Legacy Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	3,390,699	3,194,796
Legacy Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	550,162	559,289
Legacy Loans | Residential Mortgage
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	970,961	854,216
Legacy Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	2,466,658	2,463,309
Legacy Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,838,841	1,783,421
Legacy Loans | Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	92,106	131,048
Legacy Loans | Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	535,711	548,840
Acquired Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	2,062,606	2,370,047
Acquired Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,321,861	1,529,182
Acquired Loans | Commercial Loans | Commercial real estate- construction
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	89,709	61,408
Acquired Loans | Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	390,241	450,012
Acquired Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	546,723	667,532
Acquired Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	293,382	348,326
Acquired Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	1,806	1,904
Acquired Loans | Residential Mortgage
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	375,506	413,184
Acquired Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	365,239	427,681
Acquired Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	320,107	372,505
Acquired Loans | Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	24	4
Acquired Loans | Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Total loans, net of unearned income	$ 45,108	$ 55,172