Allowance for Credit Losses and Credit Quality - Schedule of Allowance for Loan Losses for Legacy and Acquired Loans, by Loan Portfolio (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	$ 144,719	$ 138,378
Transfer of balance to OREO and other	963	(2,045)
Loans charged-off	(49,939)	(30,054)
Recoveries	4,167	3,899
Allowance for loan losses at end of period	136,628	148,193
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	11,241	14,145
Balance created in purchase accounting	7,626
Reserve for unfunded commitments at end of period	21,032	11,990
Legacy Loans
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	105,569	93,808
Provision for (Reversal of) loan losses	37,835	40,516
Transfer of balance to OREO and other	0	0
Loans charged-off	(47,100)	(28,559)
Recoveries	3,042	3,124
Allowance for loan losses at end of period	99,346	108,889
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	11,241	14,145
Balance created in purchase accounting	7,626
Provision for (Reversal of) unfunded commitments	2,165	(2,155)
Reserve for unfunded commitments at end of period	21,032	11,990
Allowance on loans individually evaluated for impairment	14,091	25,210
Allowance on loans collectively evaluated for impairment	85,255	83,679
Loans, net of unearned income:
Loans, net of unearned income	13,833,816	12,413,370
Balance at end of period individually evaluated for impairment	185,171	318,466
Balance at end of period collectively evaluated for impairment	13,648,645	12,094,904
Legacy Loans | Commercial Loans | Real Estate
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	25,408	24,658
Provision for (Reversal of) loan losses	4,803	(651)
Loans charged-off	(384)	(1,598)
Recoveries	325	766
Allowance for loan losses at end of period	30,152	23,175
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	3,206	4,160
Balance created in purchase accounting	1,358
Provision for (Reversal of) unfunded commitments	1,237	(427)
Reserve for unfunded commitments at end of period	5,801	3,733
Allowance on loans individually evaluated for impairment	1,907	655
Allowance on loans collectively evaluated for impairment	28,245	22,520
Loans, net of unearned income:
Loans, net of unearned income	6,187,471	5,419,483
Balance at end of period individually evaluated for impairment	51,467	31,405
Balance at end of period collectively evaluated for impairment	6,136,004	5,388,078
Legacy Loans | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	35,434	23,283
Provision for (Reversal of) loan losses	13,345	13,201
Loans charged-off	(16,882)	(2,418)
Recoveries	474	251
Allowance for loan losses at end of period	32,371	34,317
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	3,535	3,448
Balance created in purchase accounting	4,903
Provision for (Reversal of) unfunded commitments	485	(46)
Reserve for unfunded commitments at end of period	8,923	3,402
Allowance on loans individually evaluated for impairment	4,604	8,996
Allowance on loans collectively evaluated for impairment	27,767	25,321
Loans, net of unearned income:
Loans, net of unearned income	3,497,374	3,101,472
Balance at end of period individually evaluated for impairment	39,850	58,464
Balance at end of period collectively evaluated for impairment	3,457,524	3,043,008
Legacy Loans | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	22,486	23,863
Provision for (Reversal of) loan losses	10,567	18,998
Loans charged-off	(19,802)	(14,672)
Recoveries	0	0
Allowance for loan losses at end of period	13,251	28,189
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	1,003	2,665
Balance created in purchase accounting	0
Provision for (Reversal of) unfunded commitments	269	(1,712)
Reserve for unfunded commitments at end of period	1,272	953
Allowance on loans individually evaluated for impairment	5,091	14,396
Allowance on loans collectively evaluated for impairment	8,160	13,793
Loans, net of unearned income:
Loans, net of unearned income	610,610	598,279
Balance at end of period individually evaluated for impairment	61,142	212,512
Balance at end of period collectively evaluated for impairment	549,468	385,767
Legacy Loans | Residential Mortgage
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	3,835	3,947
Provision for (Reversal of) loan losses	(500)	248
Loans charged-off	(158)	(240)
Recoveries	83	142
Allowance for loan losses at end of period	3,260	4,097
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	657	830
Balance created in purchase accounting	1,303
Provision for (Reversal of) unfunded commitments	(30)	(148)
Reserve for unfunded commitments at end of period	1,930	682
Allowance on loans individually evaluated for impairment	120	110
Allowance on loans collectively evaluated for impairment	3,140	3,987
Loans, net of unearned income:
Loans, net of unearned income	1,041,660	840,082
Balance at end of period individually evaluated for impairment	4,659	4,539
Balance at end of period collectively evaluated for impairment	1,037,001	835,543
Legacy Loans | Consumer and Other Loans
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	18,406	18,057
Provision for (Reversal of) loan losses	9,620	8,720
Loans charged-off	(9,874)	(9,631)
Recoveries	2,160	1,965
Allowance for loan losses at end of period	20,312	19,111
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	2,840	3,042
Balance created in purchase accounting	62
Provision for (Reversal of) unfunded commitments	204	178
Reserve for unfunded commitments at end of period	3,106	3,220
Allowance on loans individually evaluated for impairment	2,369	1,053
Allowance on loans collectively evaluated for impairment	17,943	18,058
Loans, net of unearned income:
Loans, net of unearned income	2,496,701	2,454,054
Balance at end of period individually evaluated for impairment	28,053	11,546
Balance at end of period collectively evaluated for impairment	2,468,648	2,442,508
Acquired Loans
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	39,150	44,570
Provision for (Reversal of) loan losses	(1,117)	(1,261)
Decrease in FDIC loss share receivable		(1,240)
Transfer of balance to OREO and other	963	(2,045)
Loans charged-off	(2,839)	(1,495)
Recoveries	1,125	775
Allowance for loan losses at end of period	37,282	39,304
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	0	0
Balance created in purchase accounting	0
Reserve for unfunded commitments at end of period	0	0
Allowance on loans individually evaluated for impairment	218	254
Allowance on loans collectively evaluated for impairment	37,064	39,050
Loans, net of unearned income:
Loans, net of unearned income	5,961,269	2,511,129
Balance at end of period individually evaluated for impairment	2,648	8,252
Balance at end of period collectively evaluated for impairment	5,476,736	1,933,830
Acquired Loans | Commercial Loans | Real Estate
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	23,574	25,979
Provision for (Reversal of) loan losses	167	(1,952)
Decrease in FDIC loss share receivable		(34)
Transfer of balance to OREO and other	879	(380)
Loans charged-off	(1,026)	(789)
Recoveries	343	102
Allowance for loan losses at end of period	23,937	22,926
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	50	177
Allowance on loans collectively evaluated for impairment	23,887	22,749
Loans, net of unearned income:
Loans, net of unearned income	2,542,154	1,261,732
Balance at end of period individually evaluated for impairment	640	5,647
Balance at end of period collectively evaluated for impairment	2,299,819	948,523
Acquired Loans | Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	3,230	2,819
Provision for (Reversal of) loan losses	(338)	216
Decrease in FDIC loss share receivable		(50)
Transfer of balance to OREO and other	(69)	(467)
Loans charged-off	(478)	0
Recoveries	131	217
Allowance for loan losses at end of period	2,476	2,735
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	48	77
Allowance on loans collectively evaluated for impairment	2,428	2,658
Loans, net of unearned income:
Loans, net of unearned income	945,711	361,525
Balance at end of period individually evaluated for impairment	196	1,926
Balance at end of period collectively evaluated for impairment	913,974	324,445
Acquired Loans | Commercial Loans | Energy-related
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	39	125
Provision for (Reversal of) loan losses	(30)	(99)
Decrease in FDIC loss share receivable		0
Transfer of balance to OREO and other	0	0
Loans charged-off	0	0
Recoveries	0	0
Allowance for loan losses at end of period	9	26
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	9	26
Loans, net of unearned income:
Loans, net of unearned income	1,003	1,362
Balance at end of period individually evaluated for impairment	0	0
Balance at end of period collectively evaluated for impairment	1,003	1,362
Acquired Loans | Residential Mortgage
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	7,412	7,841
Provision for (Reversal of) loan losses	(833)	1,017
Decrease in FDIC loss share receivable		(833)
Transfer of balance to OREO and other	2	28
Loans charged-off	(107)	0
Recoveries	65	33
Allowance for loan losses at end of period	6,539	8,086
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	44	0
Allowance on loans collectively evaluated for impairment	6,495	8,086
Loans, net of unearned income:
Loans, net of unearned income	1,983,310	430,448
Balance at end of period individually evaluated for impairment	725	0
Balance at end of period collectively evaluated for impairment	1,853,205	304,699
Acquired Loans | Consumer and Other Loans
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	4,895	7,806
Provision for (Reversal of) loan losses	(83)	(443)
Decrease in FDIC loss share receivable		(323)
Transfer of balance to OREO and other	151	(1,226)
Loans charged-off	(1,228)	(706)
Recoveries	586	423
Allowance for loan losses at end of period	4,321	5,531
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	76	0
Allowance on loans collectively evaluated for impairment	4,245	5,531
Loans, net of unearned income:
Loans, net of unearned income	489,091	456,062
Balance at end of period individually evaluated for impairment	1,087	679
Balance at end of period collectively evaluated for impairment	408,735	354,801
Receivables Acquired with Deteriorated Credit Quality | Acquired Loans
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	481,885	569,047
Receivables Acquired with Deteriorated Credit Quality | Acquired Loans | Commercial Loans | Real Estate
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	241,695	307,562
Receivables Acquired with Deteriorated Credit Quality | Acquired Loans | Commercial Loans | Commercial and industrial
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	31,541	35,154
Receivables Acquired with Deteriorated Credit Quality | Acquired Loans | Commercial Loans | Energy-related
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	0	0
Receivables Acquired with Deteriorated Credit Quality | Acquired Loans | Residential Mortgage
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	129,380	125,749
Receivables Acquired with Deteriorated Credit Quality | Acquired Loans | Consumer and Other Loans
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	$ 79,269	$ 100,582