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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	$ 125,819	$ 110,171	$ 122,291	$ 105,400
Other adjustments	(30)	(134)	(86)	(212)
Reclassification from allowance for unfunded lending-related commitments	106	(40)	(32)	(121)
Charge-offs	(6,728)	(6,097)	(10,145)	(11,571)
Recoveries	1,472	1,187	3,295	3,168
Provision for credit losses	8,952	9,269	14,268	17,692
Allowance for loan losses at period end	129,591	114,356	129,591	114,356
Allowance for unfunded lending-related commitments at period end	1,705	1,070	1,705	1,070
Allowance for credit losses at period end	131,296	115,426	131,296	115,426
Allowance for credit losses at period end, Individually evaluated for impairment	6,152	6,645	6,152	6,645
Allowance for credit losses at period end, Collectively evaluated for impairment	124,314	108,018	124,314	108,018
Loans at period end, Individually evaluated for impairment	80,072	96,836	80,072	96,836
Loans at period end, Collectively evaluated for impairment	20,245,483	17,576,641	20,245,483	17,576,641
Loans at period end, Loans held at fair value	30,173	16,294	30,173	16,294
Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	830	763	830	763
Loans at period end, Loans acquired with deteriorated credit quality	387,604	484,884	387,604	484,884
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	46,582	38,435	44,493	36,135
Other adjustments	(2)	(59)	(21)	(68)
Reclassification from allowance for unfunded lending-related commitments	92	0	0	0
Charge-offs	(913)	(721)	(1,554)	(1,392)
Recoveries	561	121	834	750
Provision for credit losses	6,038	3,878	8,606	6,229
Allowance for loan losses at period end	52,358	41,654	52,358	41,654
Allowance for unfunded lending-related commitments at period end	500	0	500	0
Allowance for credit losses at period end	52,858	41,654	52,858	41,654
Allowance for credit losses at period end, Individually evaluated for impairment	2,528	3,417	2,528	3,417
Allowance for credit losses at period end, Collectively evaluated for impairment	49,692	37,571	49,692	37,571
Loans at period end, Individually evaluated for impairment	14,469	21,173	14,469	21,173
Loans at period end, Collectively evaluated for impairment	6,381,610	5,106,634	6,381,610	5,106,634
Loans at period end, Loans held at fair value	0	0	0	0
Commercial | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	638	666	638	666
Loans at period end, Loans acquired with deteriorated credit quality	10,210	16,726	10,210	16,726
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	52,633	45,263	51,422	43,758
Other adjustments	(47)	(70)	(83)	(146)
Reclassification from allowance for unfunded lending-related commitments	14	(40)	(32)	(121)
Charge-offs	(1,985)	(502)	(2,246)	(1,173)
Recoveries	276	296	830	665
Provision for credit losses	1,448	1,877	2,448	3,841
Allowance for loan losses at period end	52,339	46,824	52,339	46,824
Allowance for unfunded lending-related commitments at period end	1,205	1,070	1,205	1,070
Allowance for credit losses at period end	53,544	47,894	53,544	47,894
Allowance for credit losses at period end, Individually evaluated for impairment	1,473	2,121	1,473	2,121
Allowance for credit losses at period end, Collectively evaluated for impairment	51,952	45,736	51,952	45,736
Loans at period end, Individually evaluated for impairment	34,690	49,284	34,690	49,284
Loans at period end, Collectively evaluated for impairment	6,221,567	5,626,752	6,221,567	5,626,752
Loans at period end, Loans held at fair value	0	0	0	0
Commercial real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	119	37	119	37
Loans at period end, Loans acquired with deteriorated credit quality	146,237	172,298	146,237	172,298
Home equity
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	12,203	12,915	11,774	12,012
Other adjustments	0	0	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(1,631)	(2,046)	(2,256)	(3,098)
Recoveries	144	71	209	119
Provision for credit losses	418	443	1,407	2,350
Allowance for loan losses at period end	11,134	11,383	11,134	11,383
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	11,134	11,383	11,134	11,383
Allowance for credit losses at period end, Individually evaluated for impairment	1,296	477	1,296	477
Allowance for credit losses at period end, Collectively evaluated for impairment	9,838	10,906	9,838	10,906
Loans at period end, Individually evaluated for impairment	9,633	8,562	9,633	8,562
Loans at period end, Collectively evaluated for impairment	679,850	752,342	679,850	752,342
Loans at period end, Loans held at fair value	0	0	0	0
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	5,530	5,164	5,714	4,734
Other adjustments	(3)	(9)	(7)	(39)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(146)	(693)	(475)	(1,186)
Recoveries	54	31	232	143
Provision for credit losses	708	912	679	1,753
Allowance for loan losses at period end	6,143	5,405	6,143	5,405
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	6,143	5,405	6,143	5,405
Allowance for credit losses at period end, Individually evaluated for impairment	764	625	764	625
Allowance for credit losses at period end, Collectively evaluated for impairment	5,306	4,720	5,306	4,720
Loans at period end, Individually evaluated for impairment	20,859	17,281	20,859	17,281
Loans at period end, Collectively evaluated for impairment	708,042	615,831	708,042	615,831
Loans at period end, Loans held at fair value	30,173	16,294	30,173	16,294
Residential real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	73	60	73	60
Loans at period end, Loans acquired with deteriorated credit quality	3,736	4,258	3,736	4,258
Premium finance receivables
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	7,559	7,205	7,625	7,233
Other adjustments	22	4	25	41
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(1,878)	(1,911)	(3,305)	(4,391)
Recoveries	404	633	1,016	1,420
Provision for credit losses	245	1,883	991	3,511
Allowance for loan losses at period end	6,352	7,814	6,352	7,814
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	6,352	7,814	6,352	7,814
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	6,352	7,814	6,352	7,814
Loans at period end, Individually evaluated for impairment	0	0	0	0
Loans at period end, Collectively evaluated for impairment	6,141,095	5,348,240	6,141,095	5,348,240
Loans at period end, Loans held at fair value	0	0	0	0
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	226,334	291,602	226,334	291,602
Consumer and other
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	1,312	1,189	1,263	1,528
Other adjustments	0	0	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(175)	(224)	(309)	(331)
Recoveries	33	35	174	71
Provision for credit losses	95	276	137	8
Allowance for loan losses at period end	1,265	1,276	1,265	1,276
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	1,265	1,276	1,265	1,276
Allowance for credit losses at period end, Individually evaluated for impairment	91	5	91	5
Allowance for credit losses at period end, Collectively evaluated for impairment	1,174	1,271	1,174	1,271
Loans at period end, Individually evaluated for impairment	421	536	421	536
Loans at period end, Collectively evaluated for impairment	113,319	126,842	113,319	126,842
Loans at period end, Loans held at fair value	0	0	0	0
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	$ 1,087	$ 0	$ 1,087	$ 0