Allowance for Credit Losses and Credit Quality - Investment in Legacy and Acquired Loans by Credit Quality Indicator (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	$ 24,100	$ 22,600
Acquired Loans
Financing Receivable, Allowance for Credit Losses
Total Loans	2,062,606	2,370,047
Acquired Loans | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(24,244)	(38,153)
Total Loans	1,321,861	1,529,182
Acquired Loans | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	13,899	8,759
Total Loans	89,709	61,408
Acquired Loans | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(12,072)	(3,084)
Total Loans	390,241	450,012
Acquired Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(24,742)	(39,335)
Total Loans	546,723	667,532
Acquired Loans | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(1,324)	(4,487)
Total Loans	293,382	348,326
Acquired Loans | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(5)	(6)
Total Loans	1,806	1,904
Acquired Loans | Residential and Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(44,706)	(53,753)
Current	749,054	859,474
30 or more days past due	36,397	35,144
Total Loans	740,745	840,865
Acquired Loans | Residential Mortgage | Residential mortgage
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(24,819)	(32,030)
Current	378,204	424,300
30 or more days past due	22,121	20,914
Total Loans	375,506	413,184
Acquired Loans | Consumer and Other Loans | Consumer - home equity
Financing Receivable, Allowance for Credit Losses
Premium/(Discount)	(15,602)	(17,323)
Current	322,658	377,021
30 or more days past due	13,051	12,807
Total Loans	320,107	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)	(4,285)	(4,392)
Current	48,168	58,141
30 or more days past due	1,225	1,423
Total Loans	45,108	55,172
Acquired Loans | Consumer and Other Loans | Consumer - other
Financing Receivable, Allowance for Credit Losses
Total Loans	44,607	54,704
Acquired Loans | Pass | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	1,274,453	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	71,157	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	384,411	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	541,724	663,571
Acquired Loans | Pass | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	275,350	323,154
Acquired Loans | Pass | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	1,811	1,910
Acquired Loans | Special Mention | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	13,357	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	1,208	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	3,275	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	3,434	11,620
Acquired Loans | Special Mention | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	5,440	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	55,561	80,003
Acquired Loans | Substandard | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	3,431	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	14,627	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	26,188	31,552
Acquired Loans | Substandard | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	11,315	27,749
Acquired Loans | Substandard | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	0	0
Acquired Loans | Doubtful | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	2,734	4,525
Acquired Loans | Doubtful | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	14	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	119	101
Acquired Loans | Doubtful | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	2,601	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	13,288,170	12,447,375
Total Loans	13,493,410	12,694,924
Legacy Loans | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Total Loans	10,055,791	9,377,399
Legacy Loans | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Current	1,008,834	740,761
Total Loans	1,010,479	740,761
Legacy Loans | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Allowance for Credit Losses
Current	1,804,289	1,775,695
Total Loans	1,815,167	1,784,624
Legacy Loans | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Allowance for Credit Losses
Current	3,277,719	3,088,207
Total Loans	3,289,284	3,097,929
Legacy Loans | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Current	3,349,442	3,158,700
Total Loans	3,390,699	3,194,796
Legacy Loans | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Current	453,357	407,434
Total Loans	550,162	559,289
Legacy Loans | Residential and Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Losses
Current	3,394,529	3,276,578
30 or more days past due	43,090	40,947
Total Loans	3,437,619	3,317,525
Legacy Loans | Residential Mortgage | Residential mortgage
Financing Receivable, Allowance for Credit Losses
Current	955,343	836,509
30 or more days past due	15,618	17,707
Total Loans	970,961	854,216
Legacy Loans | Consumer and Other Loans | Consumer - home equity
Financing Receivable, Allowance for Credit Losses
Current	1,819,163	1,768,763
30 or more days past due	19,678	14,658
Total Loans	1,838,841	1,783,421
Legacy Loans | Consumer and Other Loans | Consumer - Indirect Automobile
Financing Receivable, Allowance for Credit Losses
Current	89,590	127,054
30 or more days past due	2,516	3,994
Total Loans	92,106	131,048
Legacy Loans | Consumer and Other Loans | Consumer - Credit Card
Financing Receivable, Allowance for Credit Losses
Current	85,690	81,602
30 or more days past due	897	922
Total Loans	86,587	82,524
Legacy Loans | Consumer and Other Loans | Consumer - other
Financing Receivable, Allowance for Credit Losses
Current	444,743	462,650
30 or more days past due	4,381	3,666
Total Loans	449,124	466,316
Legacy Loans | Pass | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	9,650,074	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	997,264	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	1,740,717	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	3,251,368	3,063,470
Legacy Loans | Pass | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	3,287,383	3,112,300
Legacy Loans | Pass | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	373,342	242,123
Legacy Loans | Special Mention | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	147,904	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	7,917	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	39,772	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	8,696	8,617
Legacy Loans | Special Mention | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	42,422	29,763
Legacy Loans | Special Mention | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	49,097	80,084
Legacy Loans | Substandard | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	201,875	319,694
Legacy Loans | Substandard | Commercial Loans | Commercial real estate - construction
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	5,298	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	34,210	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	29,220	25,842
Legacy Loans | Substandard | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	58,527	35,199
Legacy Loans | Substandard | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	74,620	225,724
Legacy Loans | Doubtful | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	55,938	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	468	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	2,367	17,534
Legacy Loans | Doubtful | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Loans and Leases Receivable, before Fees, Gross	$ 53,103	$ 11,358