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, 2015	Sep. 30, 2014	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period	$ 130,131	$ 143,074
Transfer of balance to OREO	(566)	(5,378)
Transfer of balance to non-covered	0
Loans charged off	(22,234)	(21,651)
Recoveries	5,068	4,777
Allowance for loans losses at end of period	130,254	134,540
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	11,801	11,147
Reserve for unfunded commitments at end of period	14,525	12,099
Allowance for credit losses at end of period	144,779	146,639
Loans, net of unearned income:
Total covered loans	253,541		$ 444,544
Non-Covered Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period	85,367	71,899
(Reversal of) Provision for loan losses	17,886	7,840
Transfer of balance to OREO	(209)	(207)
Transfer of balance to non-covered	28,700
Loans charged off	(21,046)	(8,828)
Recoveries	5,060	4,739
Allowance for loans losses at end of period	115,758	75,443
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	11,801	11,147
(Reversal of) Provision for unfunded commitments	2,724	952
Reserve for unfunded commitments at end of period	14,525	12,099
Allowance for credit losses at end of period	130,283	87,542
Allowance on loans individually evaluated for impairment	1,716	25
Allowance on loans collectively evaluated for impairment	128,567	87,517
Balance at end of period	13,863,478	10,556,698
Loans, net of unearned income:
Balance at end of period individually evaluated for impairment	52,744	11,339
Balance at end of period collectively evaluated for impairment	13,744,595	10,525,869
Balance at end of period acquired with deteriorated credit quality	66,139	19,490
Non-Covered Loans | Commercial Loans | Real Estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period	32,630	26,126
(Reversal of) Provision for loan losses	(356)	630
Transfer of balance to OREO	174	0
Transfer of balance to non-covered	20,763
Loans charged off	(7,091)	(1,214)
Recoveries	1,543	2,209
Allowance for loans losses at end of period	47,663	27,751
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	3,370	3,071
(Reversal of) Provision for unfunded commitments	218	671
Reserve for unfunded commitments at end of period	3,588	3,742
Allowance for credit losses at end of period	51,251	31,493
Allowance on loans individually evaluated for impairment	883	25
Allowance on loans collectively evaluated for impairment	50,368	31,468
Balance at end of period	5,936,161	3,988,172
Loans, net of unearned income:
Balance at end of period individually evaluated for impairment	27,299	7,357
Balance at end of period collectively evaluated for impairment	5,874,262	3,965,754
Balance at end of period acquired with deteriorated credit quality	34,600	15,061
Non-Covered Loans | Commercial Loans | Commercial and Industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period	26,486	22,101
(Reversal of) Provision for loan losses	1,327	3,283
Transfer of balance to OREO	(169)	(130)
Transfer of balance to non-covered	1,445
Loans charged off	(1,162)	(1,097)
Recoveries	164	93
Allowance for loans losses at end of period	28,091	24,250
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	3,733	1,814
(Reversal of) Provision for unfunded commitments	(149)	1,674
Reserve for unfunded commitments at end of period	3,584	3,488
Allowance for credit losses at end of period	31,675	27,738
Allowance on loans individually evaluated for impairment	817	0
Allowance on loans collectively evaluated for impairment	30,858	27,738
Balance at end of period	3,291,552	2,398,010
Loans, net of unearned income:
Balance at end of period individually evaluated for impairment	25,231	3,276
Balance at end of period collectively evaluated for impairment	3,255,580	2,392,172
Balance at end of period acquired with deteriorated credit quality	10,741	2,562
Non-Covered Loans | Commercial Loans | Energy-Related
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period	5,999	6,878
(Reversal of) Provision for loan losses	9,339	(1,115)
Transfer of balance to OREO	0	0
Transfer of balance to non-covered	0
Loans charged off	(3)	0
Recoveries	0	0
Allowance for loans losses at end of period	15,335	5,763
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	1,596	3,043
(Reversal of) Provision for unfunded commitments	2,037	(1,427)
Reserve for unfunded commitments at end of period	3,633	1,616
Allowance for credit losses at end of period	18,968	7,379
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	18,968	7,379
Balance at end of period	719,456	839,823
Loans, net of unearned income:
Balance at end of period individually evaluated for impairment	0	0
Balance at end of period collectively evaluated for impairment	719,456	839,823
Balance at end of period acquired with deteriorated credit quality	0	0
Non-Covered Loans | Residential Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period	2,875	2,546
(Reversal of) Provision for loan losses	1,897	514
Transfer of balance to OREO	(143)	(12)
Transfer of balance to non-covered	0
Loans charged off	(283)	(607)
Recoveries	48	134
Allowance for loans losses at end of period	4,394	2,575
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	168	72
(Reversal of) Provision for unfunded commitments	650	11
Reserve for unfunded commitments at end of period	818	83
Allowance for credit losses at end of period	5,212	2,658
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	5,212	2,658
Balance at end of period	1,070,015	931,803
Loans, net of unearned income:
Balance at end of period individually evaluated for impairment	0	0
Balance at end of period collectively evaluated for impairment	1,056,797	931,643
Balance at end of period acquired with deteriorated credit quality	13,218	160
Non-Covered Loans | Consumer and Other Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period	17,377	14,248
(Reversal of) Provision for loan losses	5,679	4,528
Transfer of balance to OREO	(71)	(65)
Transfer of balance to non-covered	6,492
Loans charged off	(12,507)	(5,910)
Recoveries	3,305	2,303
Allowance for loans losses at end of period	20,275	15,104
Reserve for unfunded commitments
Reserve for unfunded commitments at beginning of period	2,934	3,147
(Reversal of) Provision for unfunded commitments	(32)	23
Reserve for unfunded commitments at end of period	2,902	3,170
Allowance for credit losses at end of period	23,177	18,274
Allowance on loans individually evaluated for impairment	16	0
Allowance on loans collectively evaluated for impairment	23,161	18,274
Balance at end of period	2,846,294	2,398,890
Loans, net of unearned income:
Balance at end of period individually evaluated for impairment	214	706
Balance at end of period collectively evaluated for impairment	2,838,500	2,396,477
Balance at end of period acquired with deteriorated credit quality	7,580	1,707
Covered Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period	44,764	71,175
(Reversal of) Provision for loan losses	1,311	4,725
(Decrease) Increase in FDIC loss share receivable	(1,342)	1,153
Transfer of balance to OREO	(357)	(5,171)
Transfer of balance to non-covered	(28,700)
Loans charged off	(1,188)	(12,823)
Recoveries	8	38
Allowance for loans losses at end of period	14,496	59,097
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 for credit losses at end of period	14,496	59,097
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	14,496	59,097
Loans, net of unearned income:
Total covered loans	253,541	524,189	444,544	[1]
Balance at end of period individually evaluated for impairment	0	0
Balance at end of period collectively evaluated for impairment	201,240	477,888
Balance at end of period acquired with deteriorated credit quality	52,301	46,301
Covered Loans | Commercial Loans
Loans, net of unearned income:
Total covered loans	55,009		220,386	[1]
Covered Loans | Commercial Loans | Real Estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period	24,072	38,772
(Reversal of) Provision for loan losses	217	2,544
(Decrease) Increase in FDIC loss share receivable	748	734
Transfer of balance to OREO	0	(1,874)
Transfer of balance to non-covered	(20,763)
Loans charged off	(1,188)	(9,578)
Recoveries	0	38
Allowance for loans losses at end of period	3,086	30,636
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	3,086	30,636
Loans, net of unearned income:
Total covered loans	43,590	247,156	189,126	[1]
Balance at end of period individually evaluated for impairment	0	0
Balance at end of period collectively evaluated for impairment	25,014	242,478
Balance at end of period acquired with deteriorated credit quality	18,576	4,678
Covered Loans | Commercial Loans | Commercial and Industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period	1,235	5,380
(Reversal of) Provision for loan losses	152	376
(Decrease) Increase in FDIC loss share receivable	59	609
Transfer of balance to OREO	(1)	(1,162)
Transfer of balance to non-covered	(1,445)
Loans charged off	0	(2,192)
Recoveries	0	0
Allowance for loans losses at end of period	0	3,011
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	0	3,011
Loans, net of unearned income:
Total covered loans	11,419	34,234	31,260	[1]
Balance at end of period individually evaluated for impairment	0	0
Balance at end of period collectively evaluated for impairment	10,852	33,614
Balance at end of period acquired with deteriorated credit quality	567	620
Covered Loans | Commercial Loans | Energy-Related
Loans, net of unearned income:
Total covered loans	0		0	[1]
Covered Loans | Residential Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period	6,286	10,889
(Reversal of) Provision for loan losses	841	971
(Decrease) Increase in FDIC loss share receivable	(277)	(2,911)
Transfer of balance to OREO	(339)	(648)
Transfer of balance to non-covered	0
Loans charged off	0	(90)
Recoveries	8	0
Allowance for loans losses at end of period	6,519	8,211
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	6,519	8,211
Loans, net of unearned income:
Total covered loans	119,926	130,976	128,024	[1]
Balance at end of period individually evaluated for impairment	0	0
Balance at end of period collectively evaluated for impairment	94,826	106,672
Balance at end of period acquired with deteriorated credit quality	25,100	24,304
Covered Loans | Consumer and Other Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period	13,171	16,134
(Reversal of) Provision for loan losses	101	834
(Decrease) Increase in FDIC loss share receivable	(1,872)	2,721
Transfer of balance to OREO	(17)	(1,487)
Transfer of balance to non-covered	(6,492)
Loans charged off	0	(963)
Recoveries	0	0
Allowance for loans losses at end of period	4,891	17,239
Reserve for unfunded commitments
Allowance on loans individually evaluated for impairment	0	0
Allowance on loans collectively evaluated for impairment	4,891	17,239
Loans, net of unearned income:
Total covered loans	78,606	111,823	$ 96,134	[1]
Balance at end of period individually evaluated for impairment	0	0
Balance at end of period collectively evaluated for impairment	70,548	95,124
Balance at end of period acquired with deteriorated credit quality	$ 8,058	$ 16,699

[1] Included as covered loans at December 31, 2014 is $174.7 million of assets whose reimbursable loss periods ended as of January 1, 2015.