Allowance for Credit Losses and Credit Quality - Investment in Legacy and Acquired Loans by Credit Quality Indicator (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	$ 26,200	$ 22,600
Acquired Loans
Financing Receivable, Allowance for Credit Losses
Total Loans	5,961,269	2,370,047
Acquired Loans | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(70,264)	(38,153)
Total Loans	3,488,868	1,529,182
Acquired Loans | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	6,256	8,759
Total Loans	244,726	61,408
Acquired Loans | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(13,825)	(3,084)
Total Loans	606,836	450,012
Acquired Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(61,173)	(39,335)
Total Loans	1,690,592	667,532
Acquired Loans | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(1,521)	(4,487)
Total Loans	945,711	348,326
Acquired Loans | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(1)	(6)
Total Loans	1,003	1,904
Acquired Loans | Residential and Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(112,344)	(53,753)
Current	2,495,518	859,474
30 or more days past due	89,227	35,144
Total Loans	2,472,401	840,865
Acquired Loans | Residential Mortgage | Residential mortgage
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(90,643)	(32,030)
Current	2,002,389	424,300
30 or more days past due	71,564	20,914
Total Loans	1,983,310	413,184
Acquired Loans | Consumer and Other Loans | Consumer - home equity
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(16,652)	(17,323)
Current	435,543	377,021
30 or more days past due	16,116	12,807
Total Loans	435,007	372,505
Acquired Loans | Consumer and Other Loans | Consumer - Indirect Automobile
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	0	(8)
Current	24	12
30 or more days past due	0	0
Total Loans	24	4
Acquired Loans | Consumer and Other Loans | Consumer - Credit Card
Financing Receivable, Allowance for Credit Losses
Total Loans	501	468
Acquired Loans | Consumer and Other Loans | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(5,049)	(4,392)
Current	57,562	58,141
30 or more days past due	1,547	1,423
Total Loans	54,060	55,172
Acquired Loans | Consumer and Other Loans | Consumer - other
Financing Receivable, Allowance for Credit Losses
Total Loans	53,559	54,704
Acquired Loans | Pass | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	3,462,339	1,461,625
Acquired Loans | Pass | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	232,608	46,498
Acquired Loans | Pass | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	589,972	426,492
Acquired Loans | Pass | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	1,712,067	663,571
Acquired Loans | Pass | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	926,837	323,154
Acquired Loans | Pass | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	855	1,910
Acquired Loans | Special Mention | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	25,244	21,159
Acquired Loans | Special Mention | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	1,590	459
Acquired Loans | Special Mention | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	9,544	7,664
Acquired Loans | Special Mention | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	11,011	11,620
Acquired Loans | Special Mention | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	3,099	1,416
Acquired Loans | Special Mention | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	0	0
Acquired Loans | Substandard | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	70,157	80,003
Acquired Loans | Substandard | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	4,259	3,118
Acquired Loans | Substandard | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	21,145	17,584
Acquired Loans | Substandard | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	28,073	31,552
Acquired Loans | Substandard | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	16,531	27,749
Acquired Loans | Substandard | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	149	0
Acquired Loans | Doubtful | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	1,392	4,525
Acquired Loans | Doubtful | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	13	2,574
Acquired Loans | Doubtful | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	0	1,356
Acquired Loans | Doubtful | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	614	101
Acquired Loans | Doubtful | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	765	494
Acquired Loans | Doubtful | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	0	0
Acquired Loans | Loss | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross		23
Acquired Loans | Loss | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross		0
Acquired Loans | Loss | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross		0
Acquired Loans | Loss | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross		23
Acquired Loans | Loss | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross		0
Acquired Loans | Loss | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross		0
Legacy Loans
Financing Receivable, Allowance for Credit Losses
Current	13,666,378	12,447,375
Total Loans	13,833,816	12,694,924
Legacy Loans | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Total Loans	10,295,455	9,377,399
Legacy Loans | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Current	1,051,124	740,761
Total Loans	1,053,245	740,761
Legacy Loans | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Allowance for Credit Losses
Current	1,792,517	1,775,695
Total Loans	1,807,670	1,784,624
Legacy Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Current	3,319,997	3,088,207
Total Loans	3,326,556	3,097,929
Legacy Loans | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Current	3,464,455	3,158,700
Total Loans	3,497,374	3,194,796
Legacy Loans | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Current	546,006	407,434
Total Loans	610,610	559,289
Legacy Loans | Residential and Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Losses
Current	3,492,279	3,276,578
30 or more days past due	46,082	40,947
Total Loans	3,538,361	3,317,525
Legacy Loans | Residential Mortgage | Residential mortgage
Financing Receivable, Allowance for Credit Losses
Current	1,023,475	836,509
30 or more days past due	18,185	17,707
Total Loans	1,041,660	854,216
Legacy Loans | Consumer and Other Loans | Consumer - home equity
Financing Receivable, Allowance for Credit Losses
Current	1,866,137	1,768,763
30 or more days past due	19,089	14,658
Total Loans	1,885,226	1,783,421
Legacy Loans | Consumer and Other Loans | Consumer - Indirect Automobile
Financing Receivable, Allowance for Credit Losses
Current	73,499	127,054
30 or more days past due	2,666	3,994
Total Loans	76,165	131,048
Legacy Loans | Consumer and Other Loans | Consumer - Credit Card
Financing Receivable, Allowance for Credit Losses
Current	87,109	81,602
30 or more days past due	845	922
Total Loans	87,954	82,524
Legacy Loans | Consumer and Other Loans | Consumer - other
Financing Receivable, Allowance for Credit Losses
Current	442,059	462,650
30 or more days past due	5,297	3,666
Total Loans	447,356	466,316
Legacy Loans | Pass | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	9,890,589	8,890,604
Legacy Loans | Pass | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	1,035,639	734,687
Legacy Loans | Pass | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	1,706,933	1,738,024
Legacy Loans | Pass | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	3,287,486	3,063,470
Legacy Loans | Pass | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	3,401,005	3,112,300
Legacy Loans | Pass | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	459,526	242,123
Legacy Loans | Special Mention | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	171,147	138,209
Legacy Loans | Special Mention | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	6,971	2,203
Legacy Loans | Special Mention | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	62,454	17,542
Legacy Loans | Special Mention | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	7,464	8,617
Legacy Loans | Special Mention | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	42,747	29,763
Legacy Loans | Special Mention | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	51,511	80,084
Legacy Loans | Substandard | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	215,312	319,694
Legacy Loans | Substandard | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	10,635	3,871
Legacy Loans | Substandard | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	38,283	29,058
Legacy Loans | Substandard | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	31,606	25,842
Legacy Loans | Substandard | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	50,862	35,199
Legacy Loans | Substandard | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	83,926	225,724
Legacy Loans | Doubtful | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	18,407	28,892
Legacy Loans | Doubtful | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	0	0
Legacy Loans | Doubtful | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	0	0
Legacy Loans | Doubtful | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	0	0
Legacy Loans | Doubtful | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	2,760	17,534
Legacy Loans | Doubtful | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	$ 15,647	$ 11,358