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	3 Months Ended
	Mar. 31, 2017	Mar. 31, 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	73	(2,459)
Loans charged off	(7,291)	(5,560)
Recoveries	1,235	1,551
Allowance for loan losses at end of period	144,890	146,557
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	11,660	14,033
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	6,566	15,908
Transfer of balance to OREO and other	0	0
Loans charged off	(7,202)	(5,389)
Recoveries	880	1,247
Allowance for loan losses at end of period	105,813	105,574
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	11,241	14,145
Provision for (Reversal of) unfunded commitments	419	(112)
Reserve for unfunded commitments at end of period	11,660	14,033
Allowance on loans individually evaluated for impairment	27,305	12,982
Allowance on loans collectively evaluated for impairment	78,508	92,592
Loans, net of unearned income:
Loans, net of unearned income	12,923,444	11,528,697
Balance at end of period individually evaluated for impairment	257,032	137,960
Balance at end of period collectively evaluated for impairment	12,666,412	11,390,737
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	1,948	1,297
Transfer of balance to OREO and other	0
Loans charged off	(95)	(1,738)
Recoveries	87	487
Allowance for loan losses at end of period	27,348	24,704
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	3,206	4,160
Provision for (Reversal of) unfunded commitments	1,377	(297)
Reserve for unfunded commitments at end of period	4,583	3,863
Allowance on loans individually evaluated for impairment	1,222	695
Allowance on loans collectively evaluated for impairment	26,126	24,009
Loans, net of unearned income:
Loans, net of unearned income	5,878,509	4,771,690
Balance at end of period individually evaluated for impairment	46,203	26,608
Balance at end of period collectively evaluated for impairment	5,832,306	4,745,082
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	1,951	(2,431)
Transfer of balance to OREO and other	0
Loans charged off	(917)	(225)
Recoveries	49	30
Allowance for loan losses at end of period	36,517	20,657
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	3,535	3,448
Provision for (Reversal of) unfunded commitments	(83)	1,952
Reserve for unfunded commitments at end of period	3,452	5,400
Allowance on loans individually evaluated for impairment	13,860	488
Allowance on loans collectively evaluated for impairment	22,657	20,169
Loans, net of unearned income:
Loans, net of unearned income	3,140,205	2,926,686
Balance at end of period individually evaluated for impairment	40,380	23,302
Balance at end of period collectively evaluated for impairment	3,099,825	2,903,384
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	490	14,533
Transfer of balance to OREO and other	0
Loans charged off	(2,845)	0
Recoveries	0	0
Allowance for loan losses at end of period	20,131	38,396
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	1,003	2,665
Provision for (Reversal of) unfunded commitments	(800)	(1,766)
Reserve for unfunded commitments at end of period	203	899
Allowance on loans individually evaluated for impairment	10,446	10,918
Allowance on loans collectively evaluated for impairment	9,685	27,478
Loans, net of unearned income:
Loans, net of unearned income	562,515	728,778
Balance at end of period individually evaluated for impairment	142,398	79,417
Balance at end of period collectively evaluated for impairment	420,117	649,361
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	(623)	(115)
Transfer of balance to OREO and other	0
Loans charged off	(22)	(14)
Recoveries	43	18
Allowance for loan losses at end of period	3,233	3,836
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	657	830
Provision for (Reversal of) unfunded commitments	(49)	(24)
Reserve for unfunded commitments at end of period	608	806
Allowance on loans individually evaluated for impairment	98	72
Allowance on loans collectively evaluated for impairment	3,135	3,764
Loans, net of unearned income:
Loans, net of unearned income	901,859	730,621
Balance at end of period individually evaluated for impairment	4,583	2,724
Balance at end of period collectively evaluated for impairment	897,276	727,897
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	2,800	2,624
Transfer of balance to OREO and other	0
Loans charged off	(3,323)	(3,412)
Recoveries	701	712
Allowance for loan losses at end of period	18,584	17,981
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	2,840	3,042
Provision for (Reversal of) unfunded commitments	(26)	23
Reserve for unfunded commitments at end of period	2,814	3,065
Allowance on loans individually evaluated for impairment	1,679	809
Allowance on loans collectively evaluated for impairment	16,905	17,172
Loans, net of unearned income:
Loans, net of unearned income	2,440,356	2,370,922
Balance at end of period individually evaluated for impairment	23,468	5,909
Balance at end of period collectively evaluated for impairment	2,416,888	2,365,013
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	(412)	(1,003)
Increase (Decrease) in FDIC loss share receivable		(258)
Transfer of balance to OREO and other	73	(2,459)
Loans charged off	(89)	(171)
Recoveries	355	304
Allowance for loan losses at end of period	39,077	40,983
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	1,504	96
Allowance on loans collectively evaluated for impairment	37,573	40,887
Loans, net of unearned income:
Loans, net of unearned income	2,208,758	2,922,547
Balance at end of period individually evaluated for impairment	12,080	7,245
Balance at end of period collectively evaluated for impairment	1,699,339	2,256,304
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	(84)	(598)
Increase (Decrease) in FDIC loss share receivable		2
Transfer of balance to OREO and other	377	(1,574)
Loans charged off	0	0
Recoveries	109	55
Allowance for loan losses at end of period	23,976	23,864
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	694	41
Allowance on loans collectively evaluated for impairment	23,282	23,823
Loans, net of unearned income:
Loans, net of unearned income	1,099,365	1,458,938
Balance at end of period individually evaluated for impairment	7,966	911
Balance at end of period collectively evaluated for impairment	832,060	1,088,713
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	63	194
Increase (Decrease) in FDIC loss share receivable		(34)
Transfer of balance to OREO and other	(350)	(343)
Loans charged off	0	(30)
Recoveries	35	108
Allowance for loan losses at end of period	2,978	2,714
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	750	5
Allowance on loans collectively evaluated for impairment	2,228	2,709
Loans, net of unearned income:
Loans, net of unearned income	315,373	447,696
Balance at end of period individually evaluated for impairment	1,810	1,582
Balance at end of period collectively evaluated for impairment	281,009	409,561
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	(26)	(25)
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	13	100
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	13	100
Loans, net of unearned income:
Loans, net of unearned income	1,108	2,884
Balance at end of period individually evaluated for impairment	0	34
Balance at end of period collectively evaluated for impairment	1,108	2,850
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	(437)	662
Increase (Decrease) in FDIC loss share receivable		(35)
Transfer of balance to OREO and other	2	(70)
Loans charged off	0	0
Recoveries	0	25
Allowance for loan losses at end of period	6,977	8,423
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	15	0
Allowance on loans collectively evaluated for impairment	6,962	8,423
Loans, net of unearned income:
Loans, net of unearned income	394,499	477,770
Balance at end of period individually evaluated for impairment	464	0
Balance at end of period collectively evaluated for impairment	278,742	340,278
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	72	(1,236)
Increase (Decrease) in FDIC loss share receivable		(191)
Transfer of balance to OREO and other	44	(472)
Loans charged off	(89)	(141)
Recoveries	211	116
Allowance for loan losses at end of period	5,133	5,882
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans individually evaluated for impairment	45	50
Allowance on loans collectively evaluated for impairment	5,088	5,832
Loans, net of unearned income:
Loans, net of unearned income	398,413	535,259
Balance at end of period individually evaluated for impairment	1,840	4,718
Balance at end of period collectively evaluated for impairment	306,420	414,902
Receivables Acquired with Deteriorated Credit Quality | Acquired Loans
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	497,339	658,998
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	259,339	369,314
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	32,554	36,553
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	115,293	137,492
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	$ 90,153	$ 115,639