Loans - Schedule of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	$ 19,795,085	$ 15,064,971
Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	13,784,323	10,906,581
Commercial Loans | Commercial real estate- construction
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	1,297,971	802,169
Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	2,414,506	2,234,636
Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	5,017,148	3,765,461
Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	4,443,085	3,543,122
Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	611,613	561,193
Residential Mortgage
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	3,024,970	1,267,400
Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	2,985,792	2,890,990
Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	2,320,233	2,155,926
Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	76,189	131,052
Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	589,370	604,012
Legacy Loans
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	13,833,816	12,694,924
Legacy Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	10,295,455	9,377,399
Legacy Loans | Commercial Loans | Commercial real estate- construction
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	1,053,245	740,761
Legacy Loans | Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	1,807,670	1,784,624
Legacy Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	3,326,556	3,097,929
Legacy Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	3,497,374	3,194,796
Legacy Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	610,610	559,289
Legacy Loans | Residential Mortgage
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	1,041,660	854,216
Legacy Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	2,496,701	2,463,309
Legacy Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	1,885,226	1,783,421
Legacy Loans | Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	76,165	131,048
Legacy Loans | Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	535,310	548,840
Acquired Loans
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	5,961,269	2,370,047
Acquired Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	3,488,868	1,529,182
Acquired Loans | Commercial Loans | Commercial real estate- construction
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	244,726	61,408
Acquired Loans | Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	606,836	450,012
Acquired Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	1,690,592	667,532
Acquired Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	945,711	348,326
Acquired Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	1,003	1,904
Acquired Loans | Residential Mortgage
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	1,983,310	413,184
Acquired Loans | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	489,091	427,681
Acquired Loans | Consumer and Other Loans | Home Equity
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	435,007	372,505
Acquired Loans | Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	24	4
Acquired Loans | Consumer and Other Loans | Other
Accounts, Notes, Loans and Financing Receivable
Loans, net of unearned income	$ 54,060	$ 55,172