Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Summary of Activity in the Allowance for Credit Losses) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	$ 94,446	$ 92,275	$ 91,705	$ 96,922
Other adjustments	(93)	(105)	(341)	(253)
Reclassification from allowance for unfunded lending-related commitments	4	(146)	(109)	(164)
Charge-offs	(6,510)	(7,859)	(10,781)	(16,876)
Recoveries	2,656	1,275	3,844	2,507
Provision for credit losses	9,701	6,813	15,886	10,117
Allowance for loan losses at period end	100,204	92,253	100,204	92,253
Allowance for unfunded lending-related commitments at period end	884	884	884	884
Allowance for credit losses at period end	101,088	93,137	101,088	93,137
Allowance for credit losses at period end, Individually evaluated for impairment	10,123	10,386	10,123	10,386
Allowance for credit losses at period end, Collectively evaluated for impairment	90,569	82,687	90,569	82,687
Loans at period end, Individually evaluated for impairment	104,577	138,735	104,577	138,735
Loans at period end, Collectively evaluated for impairment	14,949,855	13,120,311	14,949,855	13,120,311
Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	396	64	396	64
Loans at period end, Loans acquired with deteriorated credit quality	459,218	490,950	459,218	490,950
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	33,726	24,689	31,699	23,092
Other adjustments	(13)	(22)	(30)	(37)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(1,243)	(2,384)	(1,920)	(3,032)
Recoveries	285	270	655	587
Provision for credit losses	145	3,485	2,496	5,428
Allowance for loan losses at period end	32,900	26,038	32,900	26,038
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	32,900	26,038	32,900	26,038
Allowance for credit losses at period end, Individually evaluated for impairment	2,282	1,927	2,282	1,927
Allowance for credit losses at period end, Collectively evaluated for impairment	30,600	24,100	30,600	24,100
Loans at period end, Individually evaluated for impairment	11,921	12,397	11,921	12,397
Loans at period end, Collectively evaluated for impairment	4,308,690	3,615,090	4,308,690	3,615,090
Commercial | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	18	11	18	11
Loans at period end, Loans acquired with deteriorated credit quality	9,733	12,943	9,733	12,943
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	37,002	44,605	35,533	48,658
Other adjustments	(81)	(96)	(261)	(217)
Reclassification from allowance for unfunded lending-related commitments	4	(146)	(109)	(164)
Charge-offs	(856)	(2,351)	(1,861)	(6,844)
Recoveries	1,824	342	2,136	487
Provision for credit losses	4,305	(1,652)	6,760	(1,218)
Allowance for loan losses at period end	42,198	40,702	42,198	40,702
Allowance for unfunded lending-related commitments at period end	884	884	884	884
Allowance for credit losses at period end	43,082	41,586	43,082	41,586
Allowance for credit losses at period end, Individually evaluated for impairment	5,602	7,237	5,602	7,237
Allowance for credit losses at period end, Collectively evaluated for impairment	37,145	34,349	37,145	34,349
Loans at period end, Individually evaluated for impairment	65,870	100,068	65,870	100,068
Loans at period end, Collectively evaluated for impairment	4,721,897	4,188,137	4,721,897	4,188,137
Commercial real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	335	0	335	0
Loans at period end, Loans acquired with deteriorated credit quality	62,823	65,267	62,823	65,267
Home equity
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	12,664	10,966	12,500	12,611
Other adjustments	0	(1)	0	(2)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(1,847)	(730)	(2,431)	(2,997)
Recoveries	39	122	87	379
Provision for credit losses	1,432	3,561	2,132	3,927
Allowance for loan losses at period end	12,288	13,918	12,288	13,918
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	12,288	13,918	12,288	13,918
Allowance for credit losses at period end, Individually evaluated for impairment	808	636	808	636
Allowance for credit losses at period end, Collectively evaluated for impairment	11,480	13,282	11,480	13,282
Loans at period end, Individually evaluated for impairment	5,909	6,030	5,909	6,030
Loans at period end, Collectively evaluated for impairment	706,441	707,612	706,441	707,612
Home equity | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	0	0	0	0
Loans at period end, Loans acquired with deteriorated credit quality	0	0	0	0
Residential real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	4,096	4,691	4,218	5,108
Other adjustments	(5)	(2)	(8)	(4)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(923)	(689)	(1,554)	(915)
Recoveries	16	74	92	205
Provision for credit losses	1,835	(341)	2,271	(661)
Allowance for loan losses at period end	5,019	3,733	5,019	3,733
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	5,019	3,733	5,019	3,733
Allowance for credit losses at period end, Individually evaluated for impairment	1,387	484	1,387	484
Allowance for credit losses at period end, Collectively evaluated for impairment	3,589	3,196	3,589	3,196
Loans at period end, Individually evaluated for impairment	20,459	18,680	20,459	18,680
Loans at period end, Collectively evaluated for impairment	480,214	430,443	480,214	430,443
Residential real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	43	53	43	53
Loans at period end, Loans acquired with deteriorated credit quality	2,342	2,782	2,342	2,782
Premium finance receivables
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	5,992	5,582	6,513	5,583
Other adjustments	6	16	(42)	7
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(1,526)	(1,492)	(2,789)	(2,702)
Recoveries	458	314	787	635
Provision for credit losses	1,991	1,889	2,452	2,786
Allowance for loan losses at period end	6,921	6,309	6,921	6,309
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	6,921	6,309	6,921	6,309
Allowance for credit losses at period end, Individually evaluated for impairment	0	0	0	0
Allowance for credit losses at period end, Collectively evaluated for impairment	6,921	6,309	6,921	6,309
Loans at period end, Individually evaluated for impairment	0	0	0	0
Loans at period end, Collectively evaluated for impairment	4,613,563	4,020,414	4,613,563	4,020,414
Premium finance receivables | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	0	0	0	0
Loans at period end, Loans acquired with deteriorated credit quality	384,320	409,760	384,320	409,760
Consumer and other
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	966	1,742	1,242	1,870
Other adjustments	0	0	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(115)	(213)	(226)	(386)
Recoveries	34	153	87	214
Provision for credit losses	(7)	(129)	(225)	(145)
Allowance for loan losses at period end	878	1,553	878	1,553
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	878	1,553	878	1,553
Allowance for credit losses at period end, Individually evaluated for impairment	44	102	44	102
Allowance for credit losses at period end, Collectively evaluated for impairment	834	1,451	834	1,451
Loans at period end, Individually evaluated for impairment	418	1,560	418	1,560
Loans at period end, Collectively evaluated for impairment	119,050	158,615	119,050	158,615
Consumer and other | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	0	0	0	0
Loans at period end, Loans acquired with deteriorated credit quality	$ 0	$ 198	$ 0	$ 198