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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	$ 110,171	$ 94,446	$ 105,400	$ 91,705
Other adjustments	(134)	(93)	(212)	(341)
Reclassification from allowance for unfunded lending-related commitments	(40)	4	(121)	(109)
Charge-offs	(6,097)	(6,510)	(11,571)	(10,781)
Recoveries	1,187	2,656	3,168	3,844
Provision for credit losses	9,269	9,701	17,692	15,886
Allowance for loan losses at period end	114,356	100,204	114,356	100,204
Allowance for unfunded lending-related commitments at period end	1,070	884	1,070	884
Allowance for credit losses at period end	115,426	101,088	115,426	101,088
Allowance for credit losses at period end, Individually evaluated for impairment	6,645	10,123	6,645	10,123
Allowance for credit losses at period end, Collectively evaluated for impairment	108,018	90,569	108,018	90,569
Loans at period end, Individually evaluated for impairment	96,836	104,577	96,836	104,577
Loans at period end, Collectively evaluated for impairment	17,592,935	14,949,855	17,592,935	14,949,855
Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	763	396	763	396
Loans at period end, Loans acquired with deteriorated credit quality	484,884	459,218	484,884	459,218
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	38,435	33,726	36,135	31,699
Other adjustments	(59)	(13)	(68)	(30)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(721)	(1,243)	(1,392)	(1,920)
Recoveries	121	285	750	655
Provision for credit losses	3,878	145	6,229	2,496
Allowance for loan losses at period end	41,654	32,900	41,654	32,900
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	41,654	32,900	41,654	32,900
Allowance for credit losses at period end, Individually evaluated for impairment	3,417	2,282	3,417	2,282
Allowance for credit losses at period end, Collectively evaluated for impairment	37,571	30,600	37,571	30,600
Loans at period end, Individually evaluated for impairment	21,173	11,921	21,173	11,921
Loans at period end, Collectively evaluated for impairment	5,106,634	4,308,690	5,106,634	4,308,690
Commercial | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	666	18	666	18
Loans at period end, Loans acquired with deteriorated credit quality	16,726	9,733	16,726	9,733
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	45,263	37,002	43,758	35,533
Other adjustments	(70)	(81)	(146)	(261)
Reclassification from allowance for unfunded lending-related commitments	(40)	4	(121)	(109)
Charge-offs	(502)	(856)	(1,173)	(1,861)
Recoveries	296	1,824	665	2,136
Provision for credit losses	1,877	4,305	3,841	6,760
Allowance for loan losses at period end	46,824	42,198	46,824	42,198
Allowance for unfunded lending-related commitments at period end	1,070	884	1,070	884
Allowance for credit losses at period end	47,894	43,082	47,894	43,082
Allowance for credit losses at period end, Individually evaluated for impairment	2,121	5,602	2,121	5,602
Allowance for credit losses at period end, Collectively evaluated for impairment	45,736	37,145	45,736	37,145
Loans at period end, Individually evaluated for impairment	49,284	65,870	49,284	65,870
Loans at period end, Collectively evaluated for impairment	5,626,752	4,721,897	5,626,752	4,721,897
Commercial real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	37	335	37	335
Loans at period end, Loans acquired with deteriorated credit quality	172,298	62,823	172,298	62,823
Home equity
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	12,915	12,664	12,012	12,500
Other adjustments	0	0	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(2,046)	(1,847)	(3,098)	(2,431)
Recoveries	71	39	119	87
Provision for credit losses	443	1,432	2,350	2,132
Allowance for loan losses at period end	11,383	12,288	11,383	12,288
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	11,383	12,288	11,383	12,288
Allowance for credit losses at period end, Individually evaluated for impairment	477	808	477	808
Allowance for credit losses at period end, Collectively evaluated for impairment	10,906	11,480	10,906	11,480
Loans at period end, Individually evaluated for impairment	8,562	5,909	8,562	5,909
Loans at period end, Collectively evaluated for impairment	752,342	706,441	752,342	706,441
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	5,164	4,096	4,734	4,218
Other adjustments	(9)	(5)	(39)	(8)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(693)	(923)	(1,186)	(1,554)
Recoveries	31	16	143	92
Provision for credit losses	912	1,835	1,753	2,271
Allowance for loan losses at period end	5,405	5,019	5,405	5,019
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	5,405	5,019	5,405	5,019
Allowance for credit losses at period end, Individually evaluated for impairment	625	1,387	625	1,387
Allowance for credit losses at period end, Collectively evaluated for impairment	4,720	3,589	4,720	3,589
Loans at period end, Individually evaluated for impairment	17,281	20,459	17,281	20,459
Loans at period end, Collectively evaluated for impairment	632,125	480,214	632,125	480,214
Residential real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	60	43	60	43
Loans at period end, Loans acquired with deteriorated credit quality	4,258	2,342	4,258	2,342
Premium finance receivables
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	7,205	5,992	7,233	6,513
Other adjustments	4	6	41	(42)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(1,911)	(1,526)	(4,391)	(2,789)
Recoveries	633	458	1,420	787
Provision for credit losses	1,883	1,991	3,511	2,452
Allowance for loan losses at period end	7,814	6,921	7,814	6,921
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	7,814	6,921	7,814	6,921
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	7,814	6,921	7,814	6,921
Loans at period end, Individually evaluated for impairment	0	0	0	0
Loans at period end, Collectively evaluated for impairment	5,348,240	4,613,563	5,348,240	4,613,563
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	291,602	384,320	291,602	384,320
Consumer and other
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	1,189	966	1,528	1,242
Other adjustments	0	0	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(224)	(115)	(331)	(226)
Recoveries	35	34	71	87
Provision for credit losses	276	(7)	8	(225)
Allowance for loan losses at period end	1,276	878	1,276	878
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	1,276	878	1,276	878
Allowance for credit losses at period end, Individually evaluated for impairment	5	44	5	44
Allowance for credit losses at period end, Collectively evaluated for impairment	1,271	834	1,271	834
Loans at period end, Individually evaluated for impairment	536	418	536	418
Loans at period end, Collectively evaluated for impairment	126,842	119,050	126,842	119,050
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	$ 0	$ 0	$ 0