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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	$ 100,204	$ 92,253	$ 91,705	$ 96,922
Other adjustments	(153)	(335)	(494)	(588)
Reclassification from allowance for unfunded lending-related commitments	(42)	62	(151)	(102)
Charge-offs	(6,877)	(8,102)	(17,658)	(24,978)
Recoveries	1,199	1,113	5,043	3,620
Provision for credit losses	8,665	6,028	24,551	16,145
Allowance for loan losses at period end	102,996	91,019	102,996	91,019
Allowance for unfunded lending-related commitments at period end	926	822	926	822
Allowance for credit losses at period end	103,922	91,841	103,922	91,841
Allowance for credit losses at period end, Individually evaluated for impairment	8,526	6,660	8,526	6,660
Allowance for credit losses at period end, Collectively evaluated for impairment	94,435	84,962	94,435	84,962
Loans at period end, Individually evaluated for impairment	114,220	130,944	114,220	130,944
Loans at period end, Collectively evaluated for impairment	15,640,792	13,440,901	15,640,792	13,440,901
Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	961	219	961	219
Loans at period end, Loans acquired with deteriorated credit quality	561,199	480,214	561,199	480,214
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	32,900	26,038	31,699	23,092
Other adjustments	(12)	(32)	(42)	(69)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(964)	(832)	(2,884)	(3,864)
Recoveries	462	296	1,117	883
Provision for credit losses	1,604	2,442	4,100	7,870
Allowance for loan losses at period end	33,990	27,912	33,990	27,912
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	33,990	27,912	33,990	27,912
Allowance for credit losses at period end, Individually evaluated for impairment	1,881	2,296	1,881	2,296
Allowance for credit losses at period end, Collectively evaluated for impairment	31,943	25,427	31,943	25,427
Loans at period end, Individually evaluated for impairment	18,211	16,568	18,211	16,568
Loans at period end, Collectively evaluated for impairment	4,359,035	3,663,804	4,359,035	3,663,804
Commercial | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	166	189	166	189
Loans at period end, Loans acquired with deteriorated credit quality	22,939	9,299	22,939	9,299
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	42,198	40,702	35,533	48,658
Other adjustments	(85)	(265)	(346)	(482)
Reclassification from allowance for unfunded lending-related commitments	(42)	62	(151)	(102)
Charge-offs	(1,948)	(4,510)	(3,809)	(11,354)
Recoveries	213	275	2,349	762
Provision for credit losses	3,725	2,395	10,485	1,177
Allowance for loan losses at period end	44,061	38,659	44,061	38,659
Allowance for unfunded lending-related commitments at period end	926	822	926	822
Allowance for credit losses at period end	44,987	39,481	44,987	39,481
Allowance for credit losses at period end, Individually evaluated for impairment	5,832	3,507	5,832	3,507
Allowance for credit losses at period end, Collectively evaluated for impairment	38,361	35,967	38,361	35,967
Loans at period end, Individually evaluated for impairment	68,947	89,201	68,947	89,201
Loans at period end, Collectively evaluated for impairment	5,077,627	4,360,556	5,077,627	4,360,556
Commercial real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	794	7	794	7
Loans at period end, Loans acquired with deteriorated credit quality	160,992	60,618	160,992	60,618
Home equity
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	12,288	13,918	12,500	12,611
Other adjustments	0	(1)	0	(3)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(1,116)	(748)	(3,547)	(3,745)
Recoveries	42	99	129	478
Provision for credit losses	1,009	(308)	3,141	3,619
Allowance for loan losses at period end	12,223	12,960	12,223	12,960
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	12,223	12,960	12,223	12,960
Allowance for credit losses at period end, Individually evaluated for impairment	239	292	239	292
Allowance for credit losses at period end, Collectively evaluated for impairment	11,984	12,668	11,984	12,668
Loans at period end, Individually evaluated for impairment	8,365	5,922	8,365	5,922
Loans at period end, Collectively evaluated for impairment	789,100	714,136	789,100	714,136
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	5,019	3,733	4,218	5,108
Other adjustments	(6)	(2)	(14)	(6)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(1,138)	(205)	(2,692)	(1,120)
Recoveries	136	111	228	316
Provision for credit losses	575	405	2,846	(256)
Allowance for loan losses at period end	4,586	4,042	4,586	4,042
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	4,586	4,042	4,586	4,042
Allowance for credit losses at period end, Individually evaluated for impairment	544	512	544	512
Allowance for credit losses at period end, Collectively evaluated for impairment	4,042	3,530	4,042	3,530
Loans at period end, Individually evaluated for impairment	18,267	18,383	18,267	18,383
Loans at period end, Collectively evaluated for impairment	549,794	449,241	549,794	449,241
Residential real estate | 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	3,682	2,695	3,682	2,695
Premium finance receivables
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	6,921	6,309	6,513	5,583
Other adjustments	(50)	(35)	(92)	(28)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(1,595)	(1,557)	(4,384)	(4,259)
Recoveries	294	290	1,081	925
Provision for credit losses	1,511	1,260	3,963	4,046
Allowance for loan losses at period end	7,081	6,267	7,081	6,267
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	7,081	6,267	7,081	6,267
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	7,081	6,267	7,081	6,267
Loans at period end, Individually evaluated for impairment	0	0	0	0
Loans at period end, Collectively evaluated for impairment	4,733,764	4,104,695	4,733,764	4,104,695
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	373,586	407,602	373,586	407,602
Consumer and other
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	878	1,553	1,242	1,870
Other adjustments	0	0	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(116)	(250)	(342)	(636)
Recoveries	52	42	139	256
Provision for credit losses	241	(166)	16	(311)
Allowance for loan losses at period end	1,055	1,179	1,055	1,179
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	1,055	1,179	1,055	1,179
Allowance for credit losses at period end, Individually evaluated for impairment	30	53	30	53
Allowance for credit losses at period end, Collectively evaluated for impairment	1,024	1,103	1,024	1,103
Loans at period end, Individually evaluated for impairment	430	870	430	870
Loans at period end, Collectively evaluated for impairment	131,472	148,469	131,472	148,469
Consumer and other | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	1	23	1	23
Loans at period end, Loans acquired with deteriorated credit quality	$ 0	$ 0	$ 0	$ 0