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	6 Months Ended
	Jun. 30, 2017	Jun. 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	258	(967)
Loans charged-off	(19,480)	(18,555)
Recoveries	2,524	2,622
Allowance for loan losses at end of period	146,225	147,452
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	11,241	14,145
Reserve for unfunded commitments at end of period	10,462	13,826
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	18,315	28,390
Transfer of balance to OREO and other	0	0
Loans charged-off	(18,098)	(17,359)
Recoveries	1,824	2,022
Allowance for loan losses at end of period	107,610	106,861
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	11,241	14,145
Provision for (Reversal of) unfunded commitments	(779)	(319)
Reserve for unfunded commitments at end of period	10,462	13,826
Allowance on loans individually evaluated for impairment	26,920	14,485
Allowance on loans collectively evaluated for impairment	80,690	92,376
Loans, net of unearned income:
Loans, net of unearned income	13,493,410	11,984,849
Balance at end of period individually evaluated for impairment	199,468	221,358
Balance at end of period collectively evaluated for impairment	13,293,942	11,763,491
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	4,323	(715)
Loans charged-off	(256)	(1,549)
Recoveries	189	644
Allowance for loan losses at end of period	29,664	23,038
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	3,206	4,160
Provision for (Reversal of) unfunded commitments	294	(26)
Reserve for unfunded commitments at end of period	3,500	4,134
Allowance on loans individually evaluated for impairment	1,316	691
Allowance on loans collectively evaluated for impairment	28,348	22,347
Loans, net of unearned income:
Loans, net of unearned income	6,114,930	5,097,689
Balance at end of period individually evaluated for impairment	42,204	26,152
Balance at end of period collectively evaluated for impairment	6,072,726	5,071,537
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	5,181	5,874
Loans charged-off	(8,366)	(1,154)
Recoveries	269	35
Allowance for loan losses at end of period	32,518	28,038
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	3,535	3,448
Provision for (Reversal of) unfunded commitments	(46)	(60)
Reserve for unfunded commitments at end of period	3,489	3,388
Allowance on loans individually evaluated for impairment	7,192	969
Allowance on loans collectively evaluated for impairment	25,326	27,069
Loans, net of unearned income:
Loans, net of unearned income	3,390,699	3,027,590
Balance at end of period individually evaluated for impairment	36,836	34,298
Balance at end of period collectively evaluated for impairment	3,353,863	2,993,292
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	3,387	16,819
Loans charged-off	(2,845)	(7,715)
Recoveries	0	0
Allowance for loan losses at end of period	23,028	32,967
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	1,003	2,665
Provision for (Reversal of) unfunded commitments	(856)	(442)
Reserve for unfunded commitments at end of period	147	2,223
Allowance on loans individually evaluated for impairment	16,094	11,925
Allowance on loans collectively evaluated for impairment	6,934	21,042
Loans, net of unearned income:
Loans, net of unearned income	550,162	659,510
Balance at end of period individually evaluated for impairment	89,814	148,317
Balance at end of period collectively evaluated for impairment	460,348	511,193
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	(884)	264
Loans charged-off	(45)	(173)
Recoveries	77	27
Allowance for loan losses at end of period	2,983	4,065
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	657	830
Provision for (Reversal of) unfunded commitments	(89)	(23)
Reserve for unfunded commitments at end of period	568	807
Allowance on loans individually evaluated for impairment	137	100
Allowance on loans collectively evaluated for impairment	2,846	3,965
Loans, net of unearned income:
Loans, net of unearned income	970,961	794,701
Balance at end of period individually evaluated for impairment	4,631	3,451
Balance at end of period collectively evaluated for impairment	966,330	791,250
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	6,308	6,148
Loans charged-off	(6,586)	(6,768)
Recoveries	1,289	1,316
Allowance for loan losses at end of period	19,417	18,753
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	2,840	3,042
Provision for (Reversal of) unfunded commitments	(82)	232
Reserve for unfunded commitments at end of period	2,758	3,274
Allowance on loans individually evaluated for impairment	2,181	800
Allowance on loans collectively evaluated for impairment	17,236	17,953
Loans, net of unearned income:
Loans, net of unearned income	2,466,658	2,405,359
Balance at end of period individually evaluated for impairment	25,983	9,140
Balance at end of period collectively evaluated for impairment	2,440,675	2,396,219
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	(111)	(1,619)
Increase (Decrease) in FDIC loss share receivable		(797)
Transfer of balance to OREO and other	258	(967)
Loans charged-off	(1,382)	(1,196)
Recoveries	700	600
Allowance for loan losses at end of period	38,615	40,591
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	0	0
Reserve for unfunded commitments at end of period	0	0
Allowance on loans individually evaluated for impairment	1,204	68
Allowance on loans collectively evaluated for impairment	37,411	40,523
Loans, net of unearned income:
Loans, net of unearned income	2,062,606	2,737,712
Balance at end of period individually evaluated for impairment	16,592	7,877
Balance at end of period collectively evaluated for impairment	1,581,387	2,117,503
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	1,407	(1,804)
Increase (Decrease) in FDIC loss share receivable		45
Transfer of balance to OREO and other	135	(880)
Loans charged-off	(1,026)	(31)
Recoveries	145	85
Allowance for loan losses at end of period	24,235	23,394
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	402	0
Allowance on loans collectively evaluated for impairment	23,833	23,394
Loans, net of unearned income:
Loans, net of unearned income	1,026,673	1,374,312
Balance at end of period individually evaluated for impairment	10,225	1,145
Balance at end of period collectively evaluated for impairment	777,865	1,038,737
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	(46)	350
Increase (Decrease) in FDIC loss share receivable		(28)
Transfer of balance to OREO and other	(69)	323
Loans charged-off	(71)	(700)
Recoveries	77	112
Allowance for loan losses at end of period	3,121	2,876
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	678	22
Allowance on loans collectively evaluated for impairment	2,443	2,854
Loans, net of unearned income:
Loans, net of unearned income	293,382	408,219
Balance at end of period individually evaluated for impairment	2,851	2,075
Balance at end of period collectively evaluated for impairment	259,101	368,283
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	(21)	(52)
Increase (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	18	73
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	18	73
Loans, net of unearned income:
Loans, net of unearned income	1,806	2,524
Balance at end of period individually evaluated for impairment	0	0
Balance at end of period collectively evaluated for impairment	1,806	2,524
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	(578)	896
Increase (Decrease) in FDIC loss share receivable		(562)
Transfer of balance to OREO and other	2	22
Loans charged-off	(30)	0
Recoveries	65	29
Allowance for loan losses at end of period	6,871	8,226
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	32	2
Allowance on loans collectively evaluated for impairment	6,839	8,224
Loans, net of unearned income:
Loans, net of unearned income	375,506	454,361
Balance at end of period individually evaluated for impairment	660	245
Balance at end of period collectively evaluated for impairment	263,596	321,879
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	(873)	(1,009)
Increase (Decrease) in FDIC loss share receivable		(252)
Transfer of balance to OREO and other	190	(432)
Loans charged-off	(255)	(465)
Recoveries	413	374
Allowance for loan losses at end of period	4,370	6,022
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	92	44
Allowance on loans collectively evaluated for impairment	4,278	5,978
Loans, net of unearned income:
Loans, net of unearned income	365,239	498,296
Balance at end of period individually evaluated for impairment	2,856	4,412
Balance at end of period collectively evaluated for impairment	279,019	386,080
Receivables Acquired with Deteriorated Credit Quality | Acquired Loans
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	464,627	612,332
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	238,583	334,430
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	31,430	37,861
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	111,250	132,237
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	$ 83,364	$ 107,804