Allowance for Credit Losses and Credit Quality - Schedule of Allowance for Loan Losses for Covered Loans, by Loan Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Transfer of balance to OREO	$ (26)	$ (4,931)
Loans charged off	(6,528)	(2,578)
Recoveries	1,243	1,810
Balance at beginning of period	11,801	11,147
Balance at end of period	12,849	11,519
Allowance for credit losses at end of period	141,162	146,121
Balance at end of period	251,120		444,544
Non-Covered Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans losses at beginning of period	85,367	71,899
Provision for (Reversal of) loan losses	5,195	761
Transfer of balance to OREO	5	(382)
Transfer of balance to non-covered	28,700
Loans charged off	(6,319)	(2,578)
Recoveries	1,235	1,772
Allowance for loans losses at end of period	114,183	71,472
Balance at beginning of period	11,801	11,147
Provision for unfunded commitments	1,048	372
Balance at end of period	12,849	11,519
Allowance for credit losses at end of period	127,032	82,991
Allowance on loans individually evaluated for impairment	773	650
Allowance on loans collectively evaluated for impairment	113,410	70,822
Balance at end of period	12,622,341	8,976,999
Balance at end of period individually evaluated for impairment	33,363	14,324
Balance at end of period collectively evaluated for impairment	12,543,987	8,940,265
Balance at end of period acquired with deteriorated credit quality	44,991	22,410
Covered Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans losses at beginning of period	44,764	71,175
Provision for (Reversal of) loan losses	150	1,342
(Decrease) Increase in FDIC loss share receivable	(1,852)	(4,876)
Transfer of balance to OREO	(31)	(4,549)
Transfer of balance to non-covered	(28,700)
Loans charged off	(209)
Recoveries	8	38
Allowance for loans losses at end of period	14,130	63,130
Allowance for credit losses at end of period	14,130	63,130
Allowance on loans collectively evaluated for impairment	14,130	63,130
Balance at end of period	251,120	664,295	444,544
Balance at end of period collectively evaluated for impairment	217,837	612,495
Balance at end of period acquired with deteriorated credit quality	33,283	51,800
Real Estate [Member] | Non-Covered Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans losses at beginning of period	33,021	26,590
Provision for (Reversal of) loan losses	(1,024)	790
Transfer of balance to OREO	(4)	(175)
Transfer of balance to non-covered	20,982
Loans charged off	(3,235)	(599)
Recoveries	173	846
Allowance for loans losses at end of period	49,913	27,452
Balance at beginning of period	3,439	3,089
Provision for unfunded commitments	128	372
Balance at end of period	3,567	3,461
Allowance for credit losses at end of period	53,480	30,913
Allowance on loans individually evaluated for impairment	21
Allowance on loans collectively evaluated for impairment	49,892	27,452
Balance at end of period	5,132,162	3,607,349
Balance at end of period individually evaluated for impairment	20,077	7,905
Balance at end of period collectively evaluated for impairment	5,085,560	3,581,894
Balance at end of period acquired with deteriorated credit quality	26,525	17,550
Real Estate [Member] | Covered Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans losses at beginning of period	24,072	38,772
Provision for (Reversal of) loan losses	44	731
(Decrease) Increase in FDIC loss share receivable	(22)	(5,629)
Transfer of balance to OREO		(1,866)
Transfer of balance to non-covered	(20,982)
Loans charged off	(209)
Recoveries		38
Allowance for loans losses at end of period	2,903	32,046
Allowance on loans collectively evaluated for impairment	2,903	32,046
Balance at end of period	25,508	345,384
Balance at end of period collectively evaluated for impairment	22,768	340,808
Balance at end of period acquired with deteriorated credit quality	2,740	4,576
Commercial Business [Member] | Non-Covered Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans losses at beginning of period	32,094	28,515
Provision for (Reversal of) loan losses	2,205	73
Transfer of balance to OREO		(149)
Transfer of balance to non-covered	1,226
Loans charged off	(565)	(61)
Recoveries	49	33
Allowance for loans losses at end of period	35,009	28,411
Balance at beginning of period	5,260	4,839
Provision for unfunded commitments	184	91
Balance at end of period	5,444	4,930
Allowance for credit losses at end of period	40,453	33,341
Allowance on loans individually evaluated for impairment	749	464
Allowance on loans collectively evaluated for impairment	34,260	27,947
Balance at end of period	3,739,861	2,948,088
Balance at end of period individually evaluated for impairment	12,593	5,108
Balance at end of period collectively evaluated for impairment	3,724,928	2,939,935
Balance at end of period acquired with deteriorated credit quality	2,340	3,045
Commercial Business [Member] | Covered Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans losses at beginning of period	1,235	5,380
Provision for (Reversal of) loan losses		102
(Decrease) Increase in FDIC loss share receivable	(8)	(31)
Transfer of balance to OREO	(1)	(1,165)
Transfer of balance to non-covered	(1,226)
Allowance for loans losses at end of period		4,286
Allowance on loans collectively evaluated for impairment		4,286
Balance at end of period	12,132	41,695
Balance at end of period collectively evaluated for impairment	12,132	41,291
Balance at end of period acquired with deteriorated credit quality		404
Residential Mortgage [Member] | Non-Covered Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans losses at beginning of period	2,875	2,546
Provision for (Reversal of) loan losses	1,539	(278)
Transfer of balance to OREO	28	(58)
Loans charged off	(71)	(48)
Recoveries	13	13
Allowance for loans losses at end of period	4,384	2,175
Balance at beginning of period	168	72
Provision for unfunded commitments	660	2,983
Balance at end of period	828	3,055
Allowance for credit losses at end of period	5,212	5,230
Allowance on loans individually evaluated for impairment		186
Allowance on loans collectively evaluated for impairment	4,384	1,989
Balance at end of period	1,032,519	450,881
Balance at end of period individually evaluated for impairment		1,058
Balance at end of period collectively evaluated for impairment	1,019,596	449,525
Balance at end of period acquired with deteriorated credit quality	12,923	298
Residential Mortgage [Member] | Covered Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans losses at beginning of period	6,286	10,889
Provision for (Reversal of) loan losses	99	205
(Decrease) Increase in FDIC loss share receivable	24	(166)
Transfer of balance to OREO	(16)	(315)
Recoveries	8
Allowance for loans losses at end of period	6,401	10,613
Allowance on loans collectively evaluated for impairment	6,401	10,613
Balance at end of period	131,767	149,202
Balance at end of period collectively evaluated for impairment	108,761	121,385
Balance at end of period acquired with deteriorated credit quality	23,006	27,817
Consumer [Member] | Non-Covered Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans losses at beginning of period	17,377	14,248
Provision for (Reversal of) loan losses	2,475	176
Transfer of balance to OREO	(19)
Transfer of balance to non-covered	6,492
Loans charged off	(2,448)	(1,870)
Recoveries	1,000	880
Allowance for loans losses at end of period	24,877	13,434
Balance at beginning of period	2,934	3,147
Provision for unfunded commitments	76	(3,074)
Balance at end of period	3,010	73
Allowance for credit losses at end of period	27,887	13,507
Allowance on loans individually evaluated for impairment	3
Allowance on loans collectively evaluated for impairment	24,874	13,434
Balance at end of period	2,717,799	1,970,681
Balance at end of period individually evaluated for impairment	693	253
Balance at end of period collectively evaluated for impairment	2,713,903	1,968,911
Balance at end of period acquired with deteriorated credit quality	3,203	1,517
Consumer [Member] | Covered Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loans losses at beginning of period	13,171	16,134
Provision for (Reversal of) loan losses	7	304
(Decrease) Increase in FDIC loss share receivable	(1,846)	950
Transfer of balance to OREO	(14)	(1,203)
Transfer of balance to non-covered	(6,492)
Allowance for loans losses at end of period	4,826	16,185
Allowance on loans collectively evaluated for impairment	4,826	16,185
Balance at end of period	81,713	128,014
Balance at end of period collectively evaluated for impairment	74,176	109,011
Balance at end of period acquired with deteriorated credit quality	$ 7,537	$ 19,003