Loans - Schedule of Aging of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable
Acquired Impaired Loans	$ 227,870	$ 306,426
Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Acquired Impaired Loans	193,890	284,409
Residential Mortgage
Accounts, Notes, Loans and Financing Receivable
Acquired Impaired Loans	5,033	4,627
Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable
Acquired Impaired Loans	28,947	17,390
Acquired Loans
Accounts, Notes, Loans and Financing Receivable
Current or Less Than 30 days past due	5,548,469	1,841,465
Accruing	22,844	4,249
Non-accrual Loans	16,106	6,958
Discount/Premium	(108,035)	(12,811)
Acquired Impaired Loans	481,885	530,186
Total Loans	5,961,269	2,370,047
Acquired Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Total Loans	3,488,868	1,529,182
Acquired Loans | Commercial Loans | Commercial real estate - construction
Accounts, Notes, Loans and Financing Receivable
Current or Less Than 30 days past due	212,655	26,714
Accruing	3,405	0
Non-accrual Loans	1,207	1,946
Discount/Premium	(8,599)	(243)
Acquired Impaired Loans	36,058	32,991
Total Loans	244,726	61,408
Acquired Loans | Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Current or Less Than 30 days past due	498,667	326,761
Accruing	606	548
Non-accrual Loans	5,974	166
Discount/Premium	(4,775)	(3,084)
Acquired Impaired Loans	106,364	125,621
Total Loans	606,836	450,012
Acquired Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Current or Less Than 30 days past due	1,615,204	544,731
Accruing	10,783	255
Non-accrual Loans	3,869	1,055
Discount/Premium	(38,537)	(565)
Acquired Impaired Loans	99,273	122,056
Total Loans	1,690,592	667,532
Acquired Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Current or Less Than 30 days past due	911,877	314,990
Accruing	471	124
Non-accrual Loans	2,464	1,317
Discount/Premium	(642)	(837)
Acquired Impaired Loans	31,541	32,732
Total Loans	945,711	348,326
Acquired Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Current or Less Than 30 days past due	855	1,910
Accruing	149	0
Non-accrual Loans	0	0
Discount/Premium	(1)	(6)
Acquired Impaired Loans	0	0
Total Loans	1,003	1,904
Acquired Loans | Residential Mortgage | Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	2,002,389	424,300
Current or Less Than 30 days past due	1,899,229	290,031
Accruing	4,196	1,317
Non-accrual Loans	1,373	719
Discount/Premium	(50,868)	(1,835)
Acquired Impaired Loans	129,380	122,952
Total Loans	1,983,310	413,184
Acquired Loans | Consumer and Other Loans | Consumer - home equity
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	435,543	377,021
Current or Less Than 30 days past due	360,526	286,411
Accruing	2,535	1,517
Non-accrual Loans	1,024	1,395
Discount/Premium	(3,822)	(5,237)
Acquired Impaired Loans	74,744	88,419
Total Loans	435,007	372,505
Acquired Loans | Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	24	12
Current or Less Than 30 days past due	18	0
Accruing	0	0
Non-accrual Loans	0	0
Discount/Premium	0	0
Acquired Impaired Loans	6	4
Total Loans	24	4
Acquired Loans | Consumer and Other Loans | Consumer - credit card
Accounts, Notes, Loans and Financing Receivable
Current or Less Than 30 days past due	0	468
Accruing	0	0
Non-accrual Loans	0	0
Discount/Premium	0	0
Acquired Impaired Loans	501	0
Total Loans	501	468
Acquired Loans | Consumer and Other Loans | Consumer - other
Accounts, Notes, Loans and Financing Receivable
Current or Less Than 30 days past due	49,438	49,449
Accruing	699	488
Non-accrual Loans	195	360
Discount/Premium	(791)	(1,004)
Acquired Impaired Loans	4,018	5,411
Total Loans	53,559	54,704
Legacy Loans
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	13,666,378	12,447,375
Accruing	38,122	26,006
Non-accrual Loans	129,316	221,543
Total Loans	13,833,816	12,694,924
Legacy Loans | Commercial Loans
Accounts, Notes, Loans and Financing Receivable
Total Loans	10,295,455	9,377,399
Legacy Loans | Commercial Loans | Commercial real estate - construction
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	1,051,124	740,761
Accruing	690	0
Non-accrual Loans	1,431	0
Total Loans	1,053,245	740,761
Legacy Loans | Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	1,792,517	1,775,695
Accruing	4,825	1,086
Non-accrual Loans	10,328	7,843
Total Loans	1,807,670	1,784,624
Legacy Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	3,319,997	3,088,207
Accruing	1,488	1,126
Non-accrual Loans	5,071	8,596
Total Loans	3,326,556	3,097,929
Legacy Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	3,464,455	3,158,700
Accruing	7,754	4,869
Non-accrual Loans	25,165	31,227
Total Loans	3,497,374	3,194,796
Legacy Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	546,006	407,434
Accruing	2,175	1,526
Non-accrual Loans	62,429	150,329
Total Loans	610,610	559,289
Legacy Loans | Residential Mortgage | Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	1,023,475	836,509
Accruing	6,279	4,693
Non-accrual Loans	11,906	13,014
Total Loans	1,041,660	854,216
Legacy Loans | Consumer and Other Loans | Consumer - home equity
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	1,866,137	1,768,763
Accruing	8,386	6,679
Non-accrual Loans	10,703	7,979
Total Loans	1,885,226	1,783,421
Legacy Loans | Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	73,499	127,054
Accruing	1,957	2,956
Non-accrual Loans	709	1,038
Total Loans	76,165	131,048
Legacy Loans | Consumer and Other Loans | Consumer - credit card
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	87,109	81,602
Accruing	400	298
Non-accrual Loans	445	624
Total Loans	87,954	82,524
Legacy Loans | Consumer and Other Loans | Consumer - other
Accounts, Notes, Loans and Financing Receivable
Current or less than 30 days past due	442,059	462,650
Accruing	4,168	2,773
Non-accrual Loans	1,129	893
Total Loans	447,356	466,316
30-59 days | Acquired Loans
Accounts, Notes, Loans and Financing Receivable
Accruing	20,877	2,586
30-59 days | Acquired Loans | Commercial Loans | Commercial real estate - construction
Accounts, Notes, Loans and Financing Receivable
Accruing	3,405	0
30-59 days | Acquired Loans | Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Accruing	606	493
30-59 days | Acquired Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Accruing	9,847	223
30-59 days | Acquired Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Accruing	469	73
30-59 days | Acquired Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Accruing	149	0
30-59 days | Acquired Loans | Residential Mortgage | Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Accruing	3,663	328
30-59 days | Acquired Loans | Consumer and Other Loans | Consumer - home equity
Accounts, Notes, Loans and Financing Receivable
Accruing	2,142	1,078
30-59 days | Acquired Loans | Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
30-59 days | Acquired Loans | Consumer and Other Loans | Consumer - credit card
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
30-59 days | Acquired Loans | Consumer and Other Loans | Consumer - other
Accounts, Notes, Loans and Financing Receivable
Accruing	596	391
30-59 days | Legacy Loans
Accounts, Notes, Loans and Financing Receivable
Accruing	26,138	18,026
30-59 days | Legacy Loans | Commercial Loans | Commercial real estate - construction
Accounts, Notes, Loans and Financing Receivable
Accruing	690	0
30-59 days | Legacy Loans | Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Accruing	4,694	959
30-59 days | Legacy Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Accruing	289	902
30-59 days | Legacy Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Accruing	4,797	3,999
30-59 days | Legacy Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
30-59 days | Legacy Loans | Residential Mortgage | Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Accruing	4,030	2,012
30-59 days | Legacy Loans | Consumer and Other Loans | Consumer - home equity
Accounts, Notes, Loans and Financing Receivable
Accruing	6,184	5,249
30-59 days | Legacy Loans | Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Accruing	1,683	2,551
30-59 days | Legacy Loans | Consumer and Other Loans | Consumer - credit card
Accounts, Notes, Loans and Financing Receivable
Accruing	256	199
30-59 days | Legacy Loans | Consumer and Other Loans | Consumer - other
Accounts, Notes, Loans and Financing Receivable
Accruing	3,515	2,155
60-89 days | Acquired Loans
Accounts, Notes, Loans and Financing Receivable
Accruing	1,765	1,381
60-89 days | Acquired Loans | Commercial Loans | Commercial real estate - construction
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
60-89 days | Acquired Loans | Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Accruing	0	55
60-89 days | Acquired Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Accruing	936	0
60-89 days | Acquired Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Accruing	0	51
60-89 days | Acquired Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
60-89 days | Acquired Loans | Residential Mortgage | Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Accruing	333	989
60-89 days | Acquired Loans | Consumer and Other Loans | Consumer - home equity
Accounts, Notes, Loans and Financing Receivable
Accruing	393	189
60-89 days | Acquired Loans | Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
60-89 days | Acquired Loans | Consumer and Other Loans | Consumer - credit card
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
60-89 days | Acquired Loans | Consumer and Other Loans | Consumer - other
Accounts, Notes, Loans and Financing Receivable
Accruing	103	97
60-89 days | Legacy Loans
Accounts, Notes, Loans and Financing Receivable
Accruing	9,993	6,876
60-89 days | Legacy Loans | Commercial Loans | Commercial real estate - construction
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
60-89 days | Legacy Loans | Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Accruing	131	127
60-89 days | Legacy Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Accruing	1,063	224
60-89 days | Legacy Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Accruing	1,584	870
60-89 days | Legacy Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Accruing	2,175	1,526
60-89 days | Legacy Loans | Residential Mortgage | Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Accruing	1,785	1,577
60-89 days | Legacy Loans | Consumer and Other Loans | Consumer - home equity
Accounts, Notes, Loans and Financing Receivable
Accruing	2,202	1,430
60-89 days | Legacy Loans | Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Accruing	262	405
60-89 days | Legacy Loans | Consumer and Other Loans | Consumer - credit card
Accounts, Notes, Loans and Financing Receivable
Accruing	144	99
60-89 days | Legacy Loans | Consumer and Other Loans | Consumer - other
Accounts, Notes, Loans and Financing Receivable
Accruing	647	618
Greater than 90 days | Acquired Loans
Accounts, Notes, Loans and Financing Receivable
Accruing	202	282
Greater than 90 days | Acquired Loans | Commercial Loans | Commercial real estate - construction
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
Greater than 90 days | Acquired Loans | Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
Greater than 90 days | Acquired Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Accruing	0	32
Greater than 90 days | Acquired Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Accruing	2	0
Greater than 90 days | Acquired Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
Greater than 90 days | Acquired Loans | Residential Mortgage | Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Accruing	200	0
Greater than 90 days | Acquired Loans | Consumer and Other Loans | Consumer - home equity
Accounts, Notes, Loans and Financing Receivable
Accruing	0	250
Greater than 90 days | Acquired Loans | Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
Greater than 90 days | Acquired Loans | Consumer and Other Loans | Consumer - credit card
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
Greater than 90 days | Acquired Loans | Consumer and Other Loans | Consumer - other
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
Greater than 90 days | Legacy Loans
Accounts, Notes, Loans and Financing Receivable
Accruing	1,991	1,104
Greater than 90 days | Legacy Loans | Commercial Loans | Commercial real estate - construction
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
Greater than 90 days | Legacy Loans | Commercial Loans | Commercial real estate- owner-occupied
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
Greater than 90 days | Legacy Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Accounts, Notes, Loans and Financing Receivable
Accruing	136	0
Greater than 90 days | Legacy Loans | Commercial Loans | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Accruing	1,373	0
Greater than 90 days | Legacy Loans | Commercial Loans | Energy-related
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
Greater than 90 days | Legacy Loans | Residential Mortgage | Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Accruing	464	1,104
Greater than 90 days | Legacy Loans | Consumer and Other Loans | Consumer - home equity
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
Greater than 90 days | Legacy Loans | Consumer and Other Loans | Consumer - indirect automobile
Accounts, Notes, Loans and Financing Receivable
Accruing	12	0
Greater than 90 days | Legacy Loans | Consumer and Other Loans | Consumer - credit card
Accounts, Notes, Loans and Financing Receivable
Accruing	0	0
Greater than 90 days | Legacy Loans | Consumer and Other Loans | Consumer - other
Accounts, Notes, Loans and Financing Receivable
Accruing	$ 6	$ 0