Allowance for Credit Losses and Credit Quality - Schedule of Allowance for Loan Losses by Loan Portfolio (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 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	(109)	(26)
Loans charged off	(7,910)	(6,528)
Recoveries	1,551	1,243
Allowance for loan losses at end of period	146,557	128,313
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	14,145	11,801
Reserve for unfunded commitments at end of period	14,033	12,849
Loans, net of unearned income:
Total covered loans	220,492		$ 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	15,908	4,177
Loans charged off	(5,389)	(2,669)
Recoveries	1,247	1,091
Allowance for loan losses at end of period	105,574	78,773
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	14,145	11,801
Provision for (Reversal of) unfunded commitments	(112)	1,048
Reserve for unfunded commitments at end of period	14,033	12,849
Allowance on loans individually evaluated for impairment	12,982	773
Allowance on loans collectively evaluated for impairment	92,592	78,000
Loans, net of unearned income:
Total covered loans	11,528,697	9,894,869
Balance at end of period individually evaluated for impairment	137,960	33,363
Balance at end of period collectively evaluated for impairment	11,390,737	9,861,506
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	1,297	(1,231)
Loans charged off	(1,738)	0
Recoveries	487	173
Allowance for loan losses at end of period	24,704	25,694
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	4,160	3,370
Provision for (Reversal of) unfunded commitments	(297)	125
Reserve for unfunded commitments at end of period	3,863	3,495
Allowance on loans individually evaluated for impairment	695	21
Allowance on loans collectively evaluated for impairment	24,009	25,673
Loans, net of unearned income:
Total covered loans	4,771,690	3,845,551
Balance at end of period individually evaluated for impairment	26,608	20,077
Balance at end of period collectively evaluated for impairment	4,745,082	3,825,474
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	(2,431)	460
Loans charged off	(225)	(460)
Recoveries	30	49
Allowance for loan losses at end of period	20,657	24,504
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	3,448	3,733
Provision for (Reversal of) unfunded commitments	1,952	(347)
Reserve for unfunded commitments at end of period	5,400	3,386
Allowance on loans individually evaluated for impairment	488	749
Allowance on loans collectively evaluated for impairment	20,169	23,755
Loans, net of unearned income:
Total covered loans	2,926,686	2,496,258
Balance at end of period individually evaluated for impairment	23,302	12,593
Balance at end of period collectively evaluated for impairment	2,903,384	2,483,665
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	14,533	1,722
Loans charged off	0	0
Recoveries	0	0
Allowance for loan losses at end of period	38,396	7,671
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	2,665	1,596
Provision for (Reversal of) unfunded commitments	(1,766)	534
Reserve for unfunded commitments at end of period	899	2,130
Allowance on loans individually evaluated for impairment	10,918	0
Allowance on loans collectively evaluated for impairment	27,478	7,671
Loans, net of unearned income:
Total covered loans	728,778	815,281
Balance at end of period individually evaluated for impairment	79,417	0
Balance at end of period collectively evaluated for impairment	649,361	815,281
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	(115)	1,563
Loans charged off	(14)	(48)
Recoveries	18	12
Allowance for loan losses at end of period	3,836	4,205
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	830	168
Provision for (Reversal of) unfunded commitments	(24)	660
Reserve for unfunded commitments at end of period	806	828
Allowance on loans individually evaluated for impairment	72	0
Allowance on loans collectively evaluated for impairment	3,764	4,205
Loans, net of unearned income:
Total covered loans	730,621	553,815
Balance at end of period individually evaluated for impairment	2,724	0
Balance at end of period collectively evaluated for impairment	727,897	553,815
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	2,624	1,663
Loans charged off	(3,412)	(2,161)
Recoveries	712	857
Allowance for loan losses at end of period	17,981	16,699
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	3,042	2,934
Provision for (Reversal of) unfunded commitments	23	76
Reserve for unfunded commitments at end of period	3,065	3,010
Allowance on loans individually evaluated for impairment	809	3
Allowance on loans collectively evaluated for impairment	17,172	16,696
Loans, net of unearned income:
Total covered loans	2,370,922	2,183,964
Balance at end of period individually evaluated for impairment	5,909	693
Balance at end of period collectively evaluated for impairment	2,365,013	2,183,271
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	(1,003)	1,168
Increase (Decrease) in FDIC loss share receivable	(258)	(1,852)
Transfer of balance to OREO	(109)	(26)
Loans charged off	(2,521)	(3,859)
Recoveries	304	152
Allowance for loan losses at end of period	40,983	49,540
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	96	0
Allowance on loans collectively evaluated for impairment	40,887	49,540
Loans, net of unearned income:
Total covered loans	2,922,547	2,978,592
Balance at end of period individually evaluated for impairment	7,245	0
Balance at end of period collectively evaluated for impairment	2,256,304	2,140,954
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	(598)	242
Increase (Decrease) in FDIC loss share receivable	2	(22)
Transfer of balance to OREO	(15)	(4)
Loans charged off	(1,808)	(3,445)
Recoveries	304	0
Allowance for loan losses at end of period	23,864	26,720
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	41	0
Allowance on loans collectively evaluated for impairment	23,823	26,720
Loans, net of unearned income:
Total covered loans	1,458,938	1,277,395
Balance at end of period individually evaluated for impairment	911	0
Balance at end of period collectively evaluated for impairment	1,088,713	790,520
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	194	47
Increase (Decrease) in FDIC loss share receivable	(34)	(8)
Transfer of balance to OREO	(21)	(1)
Loans charged off	(244)	(105)
Recoveries	0	0
Allowance for loan losses at end of period	2,714	3,198
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	5	0
Allowance on loans collectively evaluated for impairment	2,709	3,198
Loans, net of unearned income:
Total covered loans	447,696	471,048
Balance at end of period individually evaluated for impairment	1,582	0
Balance at end of period collectively evaluated for impairment	409,561	413,972
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	(25)	(16)
Increase (Decrease) in FDIC loss share receivable	0	0
Transfer of balance to OREO	0	0
Loans charged off	0	0
Recoveries	0	0
Allowance for loan losses at end of period	100	35
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	100	35
Loans, net of unearned income:
Total covered loans	2,884	4,130
Balance at end of period individually evaluated for impairment	34	0
Balance at end of period collectively evaluated for impairment	2,850	4,130
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	662	75
Increase (Decrease) in FDIC loss share receivable	(35)	24
Transfer of balance to OREO	(45)	14
Loans charged off	0	(22)
Recoveries	0	8
Allowance for loan losses at end of period	8,423	6,583
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	8,423	6,583
Loans, net of unearned income:
Total covered loans	477,770	610,471
Balance at end of period individually evaluated for impairment	0	0
Balance at end of period collectively evaluated for impairment	340,278	448,220
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	(1,236)	820
Increase (Decrease) in FDIC loss share receivable	(191)	(1,846)
Transfer of balance to OREO	(28)	(35)
Loans charged off	(469)	(287)
Recoveries	0	144
Allowance for loan losses at end of period	5,882	13,004
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	50	0
Allowance on loans collectively evaluated for impairment	5,832	13,004
Loans, net of unearned income:
Total covered loans	535,259	615,548
Balance at end of period individually evaluated for impairment	4,718	0
Balance at end of period collectively evaluated for impairment	414,902	484,112
Receivables Acquired With Deteriorated Credit Quality | Covered Loans
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	658,998	837,638
Receivables Acquired With Deteriorated Credit Quality | Covered Loans | Commercial Loans | Real Estate
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	369,314	486,875
Receivables Acquired With Deteriorated Credit Quality | Covered Loans | Commercial Loans | Commercial and Industrial
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	36,553	57,076
Receivables Acquired With Deteriorated Credit Quality | Covered 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 | Covered Loans | Residential Mortgage
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	137,492	162,251
Receivables Acquired With Deteriorated Credit Quality | Covered Loans | Consumer and Other Loans
Loans, net of unearned income:
Balance at end of period acquired with deteriorated credit quality	$ 115,639	$ 131,436