Allowance for Credit Losses and Credit Quality - Schedule of Allowance for Loan Losses by Loan Portfolio (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	$ 138,378	$ 130,131
Transfer of balance to OREO and other	2,045	9,768
Loans charged off	(30,054)	(13,025)
Recoveries	3,899	5,061
Allowance for loan losses at end of period	148,193	130,254
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	14,145	11,801
Reserve for unfunded commitments at end of period	11,990	14,525
Loans, net of unearned income:
Loans, net of unearned income	202,211		$ 229,217
Non-Covered Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	93,808	76,174
Provision for (Reversal of) loan losses	40,516	17,743
Loans charged off	(28,559)	(12,073)
Recoveries	3,124	4,556
Allowance for loan losses at end of period	108,889	86,400
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	14,145	11,801
Provision for (Reversal of) unfunded commitments	(2,155)	2,724
Reserve for unfunded commitments at end of period	11,990	14,525
Allowance on loans individually evaluated for impairment	25,210	1,700
Allowance on loans collectively evaluated for impairment	83,679	84,700
Loans, net of unearned income:
Loans, net of unearned income	12,413,370	10,779,258
Balance at end of period individually evaluated for impairment	318,466	52,146
Balance at end of period collectively evaluated for impairment	12,094,904	10,727,112
Non-Covered Loans | Commercial Loans | Real Estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	24,658	26,752
Provision for (Reversal of) loan losses	(651)	(947)
Loans charged off	(1,598)	(2,009)
Recoveries	766	1,535
Allowance for loan losses at end of period	23,175	25,331
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	4,160	3,370
Provision for (Reversal of) unfunded commitments	(427)	218
Reserve for unfunded commitments at end of period	3,733	3,588
Allowance on loans individually evaluated for impairment	655	883
Allowance on loans collectively evaluated for impairment	22,520	24,448
Loans, net of unearned income:
Loans, net of unearned income	5,419,483	4,321,723
Balance at end of period individually evaluated for impairment	31,405	26,463
Balance at end of period collectively evaluated for impairment	5,388,078	4,295,260
Non-Covered Loans | Commercial Loans | Commercial and Industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	23,283	24,455
Provision for (Reversal of) loan losses	13,201	1,308
Loans charged off	(2,418)	(1,048)
Recoveries	251	49
Allowance for loan losses at end of period	34,317	24,764
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	3,448	3,733
Provision for (Reversal of) unfunded commitments	(46)	(149)
Reserve for unfunded commitments at end of period	3,402	3,584
Allowance on loans individually evaluated for impairment	8,996	817
Allowance on loans collectively evaluated for impairment	25,321	23,947
Loans, net of unearned income:
Loans, net of unearned income	3,101,472	2,779,503
Balance at end of period individually evaluated for impairment	58,464	22,076
Balance at end of period collectively evaluated for impairment	3,043,008	2,757,427
Non-Covered Loans | Commercial Loans | Energy-Related
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	23,863	5,949
Provision for (Reversal of) loan losses	18,998	9,269
Loans charged off	(14,672)	0
Recoveries	0	0
Allowance for loan losses at end of period	28,189	15,218
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	2,665	1,596
Provision for (Reversal of) unfunded commitments	(1,712)	2,037
Reserve for unfunded commitments at end of period	953	3,633
Allowance on loans individually evaluated for impairment	14,396	0
Allowance on loans collectively evaluated for impairment	13,793	15,218
Loans, net of unearned income:
Loans, net of unearned income	598,279	713,935
Balance at end of period individually evaluated for impairment	212,512	3,409
Balance at end of period collectively evaluated for impairment	385,767	710,526
Non-Covered Loans | Residential Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	3,947	2,678
Provision for (Reversal of) loan losses	248	1,474
Loans charged off	(240)	(224)
Recoveries	142	44
Allowance for loan losses at end of period	4,097	3,972
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	830	168
Provision for (Reversal of) unfunded commitments	(148)	650
Reserve for unfunded commitments at end of period	682	818
Allowance on loans individually evaluated for impairment	110	0
Allowance on loans collectively evaluated for impairment	3,987	3,972
Loans, net of unearned income:
Loans, net of unearned income	840,082	660,543
Balance at end of period individually evaluated for impairment	4,539	0
Balance at end of period collectively evaluated for impairment	835,543	660,543
Non-Covered Loans | Consumer and Other Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	18,057	16,340
Provision for (Reversal of) loan losses	8,720	6,639
Loans charged off	(9,631)	(8,792)
Recoveries	1,965	2,928
Allowance for loan losses at end of period	19,111	17,115
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	3,042	2,934
Provision for (Reversal of) unfunded commitments	178	(32)
Reserve for unfunded commitments at end of period	3,220	2,902
Allowance on loans individually evaluated for impairment	1,053	0
Allowance on loans collectively evaluated for impairment	18,058	17,115
Loans, net of unearned income:
Loans, net of unearned income	2,454,054	2,303,554
Balance at end of period individually evaluated for impairment	11,546	198
Balance at end of period collectively evaluated for impairment	2,442,508	2,303,356
Covered Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	44,570	53,957
Provision for (Reversal of) loan losses	(1,261)	1,454
Decrease in FDIC loss share receivable	(1,240)	(1,342)
Transfer of balance to OREO and other	(2,045)	(9,768)
Loans charged off	(1,495)	(952)
Recoveries	775	505
Allowance for loan losses at end of period	39,304	43,854
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	254	16
Allowance on loans collectively evaluated for impairment	39,050	43,838
Loans, net of unearned income:
Loans, net of unearned income	2,511,129	3,337,761
Balance at end of period individually evaluated for impairment	8,252	598
Balance at end of period collectively evaluated for impairment	1,933,830	2,555,360
Covered Loans | Receivables Acquired With Deteriorated Credit Quality
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	569,047	781,803
Covered Loans | Commercial Loans | Real Estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	25,979	29,949
Provision for (Reversal of) loan losses	(1,952)	809
Decrease in FDIC loss share receivable	(34)	748
Transfer of balance to OREO and other	(380)	(6,096)
Loans charged off	(789)	0
Recoveries	102	8
Allowance for loan losses at end of period	22,926	25,418
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	177	0
Allowance on loans collectively evaluated for impairment	22,749	25,418
Loans, net of unearned income:
Loans, net of unearned income	1,261,732	1,658,028
Balance at end of period individually evaluated for impairment	5,647	0
Balance at end of period collectively evaluated for impairment	948,523	1,205,439
Covered Loans | Commercial Loans | Real Estate | Receivables Acquired With Deteriorated Credit Quality
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	307,562	452,589
Covered Loans | Commercial Loans | Commercial and Industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	2,819	3,265
Provision for (Reversal of) loan losses	216	174
Decrease in FDIC loss share receivable	(50)	59
Transfer of balance to OREO and other	(467)	(282)
Loans charged off	0	(8)
Recoveries	217	116
Allowance for loan losses at end of period	2,735	3,324
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	77	0
Allowance on loans collectively evaluated for impairment	2,658	3,324
Loans, net of unearned income:
Loans, net of unearned income	361,525	523,468
Balance at end of period individually evaluated for impairment	1,926	142
Balance at end of period collectively evaluated for impairment	324,445	465,514
Covered Loans | Commercial Loans | Commercial and Industrial | Receivables Acquired With Deteriorated Credit Quality
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	35,154	57,812
Covered Loans | Commercial Loans | Energy-Related
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	125	51
Provision for (Reversal of) loan losses	(99)	67
Decrease in FDIC loss share receivable	0	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	26	118
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	26	118
Loans, net of unearned income:
Loans, net of unearned income	1,362	5,521
Balance at end of period individually evaluated for impairment	0	0
Balance at end of period collectively evaluated for impairment	1,362	5,521
Covered Loans | Commercial Loans | Energy-Related | Receivables Acquired With Deteriorated Credit Quality
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	0	0
Covered Loans | Residential Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	7,841	6,484
Provision for (Reversal of) loan losses	1,017	1,263
Decrease in FDIC loss share receivable	(833)	(277)
Transfer of balance to OREO and other	28	(472)
Loans charged off	0	(59)
Recoveries	33	4
Allowance for loan losses at end of period	8,086	6,943
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	8,086	6,943
Loans, net of unearned income:
Loans, net of unearned income	430,448	529,398
Balance at end of period individually evaluated for impairment	0	0
Balance at end of period collectively evaluated for impairment	304,699	385,854
Covered Loans | Residential Mortgage | Receivables Acquired With Deteriorated Credit Quality
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	125,749	143,544
Covered Loans | Consumer and Other Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	7,806	14,208
Provision for (Reversal of) loan losses	(443)	(859)
Decrease in FDIC loss share receivable	(323)	(1,872)
Transfer of balance to OREO and other	(1,226)	(2,918)
Loans charged off	(706)	(885)
Recoveries	423	377
Allowance for loan losses at end of period	5,531	8,051
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	0	16
Allowance on loans collectively evaluated for impairment	5,531	8,035
Loans, net of unearned income:
Loans, net of unearned income	456,062	621,346
Balance at end of period individually evaluated for impairment	679	456
Balance at end of period collectively evaluated for impairment	354,801	493,032
Covered Loans | Consumer and Other Loans | Receivables Acquired With Deteriorated Credit Quality
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	$ 100,582	$ 127,858