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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	$ 114,356	$ 100,204	$ 105,400	$ 91,705
Other adjustments	(112)	(153)	(324)	(494)
Reclassification from allowance for unfunded lending-related commitments	(579)	(42)	(700)	(151)
Charge-offs	(6,907)	(6,877)	(18,478)	(17,658)
Recoveries	1,194	1,199	4,362	5,043
Provision for credit losses	9,741	8,665	27,433	24,551
Allowance for loan losses at period end	117,693	102,996	117,693	102,996
Allowance for unfunded lending-related commitments at period end	1,648	926	1,648	926
Allowance for credit losses at period end	119,341	103,922	119,341	103,922
Allowance for credit losses at period end, Individually evaluated for impairment	7,367	8,526	7,367	8,526
Allowance for credit losses at period end, Collectively evaluated for impairment	111,150	94,435	111,150	94,435
Loans at period end, Individually evaluated for impairment	91,374	114,220	91,374	114,220
Loans at period end, Collectively evaluated for impairment	18,569,768	15,640,792	18,569,768	15,640,792
Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	824	961	824	961
Loans at period end, Loans acquired with deteriorated credit quality	440,119	561,199	440,119	561,199
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	41,654	32,900	36,135	31,699
Other adjustments	(35)	(12)	(103)	(42)
Reclassification from allowance for unfunded lending-related commitments	(500)	0	(500)	0
Charge-offs	(3,469)	(964)	(4,861)	(2,884)
Recoveries	176	462	926	1,117
Provision for credit losses	5,212	1,604	11,441	4,100
Allowance for loan losses at period end	43,038	33,990	43,038	33,990
Allowance for unfunded lending-related commitments at period end	500	0	500	0
Allowance for credit losses at period end	43,538	33,990	43,538	33,990
Allowance for credit losses at period end, Individually evaluated for impairment	2,554	1,881	2,554	1,881
Allowance for credit losses at period end, Collectively evaluated for impairment	40,252	31,943	40,252	31,943
Loans at period end, Individually evaluated for impairment	19,133	18,211	19,133	18,211
Loans at period end, Collectively evaluated for impairment	5,916,432	4,359,035	5,916,432	4,359,035
Commercial | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	732	166	732	166
Loans at period end, Loans acquired with deteriorated credit quality	15,979	22,939	15,979	22,939
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	46,824	42,198	43,758	35,533
Other adjustments	(57)	(85)	(203)	(346)
Reclassification from allowance for unfunded lending-related commitments	(79)	(42)	(200)	(151)
Charge-offs	(382)	(1,948)	(1,555)	(3,809)
Recoveries	364	213	1,029	2,349
Provision for credit losses	1,678	3,725	5,519	10,485
Allowance for loan losses at period end	48,348	44,061	48,348	44,061
Allowance for unfunded lending-related commitments at period end	1,148	926	1,148	926
Allowance for credit losses at period end	49,496	44,987	49,496	44,987
Allowance for credit losses at period end, Individually evaluated for impairment	2,491	5,832	2,491	5,832
Allowance for credit losses at period end, Collectively evaluated for impairment	46,983	38,361	46,983	38,361
Loans at period end, Individually evaluated for impairment	45,290	68,947	45,290	68,947
Loans at period end, Collectively evaluated for impairment	5,707,277	5,077,627	5,707,277	5,077,627
Commercial real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	22	794	22	794
Loans at period end, Loans acquired with deteriorated credit quality	156,117	160,992	156,117	160,992
Home equity
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	11,383	12,288	12,012	12,500
Other adjustments	0	0	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(574)	(1,116)	(3,672)	(3,547)
Recoveries	65	42	184	129
Provision for credit losses	810	1,009	3,160	3,141
Allowance for loan losses at period end	11,684	12,223	11,684	12,223
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	11,684	12,223	11,684	12,223
Allowance for credit losses at period end, Individually evaluated for impairment	964	239	964	239
Allowance for credit losses at period end, Collectively evaluated for impairment	10,720	11,984	10,720	11,984
Loans at period end, Individually evaluated for impairment	9,309	8,365	9,309	8,365
Loans at period end, Collectively evaluated for impairment	733,559	789,100	733,559	789,100
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	5,405	5,019	4,734	4,218
Other adjustments	(10)	(6)	(49)	(14)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(134)	(1,138)	(1,320)	(2,692)
Recoveries	61	136	204	228
Provision for credit losses	781	575	2,534	2,846
Allowance for loan losses at period end	6,103	4,586	6,103	4,586
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	6,103	4,586	6,103	4,586
Allowance for credit losses at period end, Individually evaluated for impairment	1,166	544	1,166	544
Allowance for credit losses at period end, Collectively evaluated for impairment	4,867	4,042	4,867	4,042
Loans at period end, Individually evaluated for impairment	17,040	18,267	17,040	18,267
Loans at period end, Collectively evaluated for impairment	642,633	549,794	642,633	549,794
Residential real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	70	0	70	0
Loans at period end, Loans acquired with deteriorated credit quality	3,925	3,682	3,925	3,682
Premium finance receivables
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	7,814	6,921	7,233	6,513
Other adjustments	(10)	(50)	31	(92)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(1,959)	(1,595)	(6,350)	(4,384)
Recoveries	456	294	1,876	1,081
Provision for credit losses	974	1,511	4,485	3,963
Allowance for loan losses at period end	7,275	7,081	7,275	7,081
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	7,275	7,081	7,275	7,081
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	7,275	7,081	7,275	7,081
Loans at period end, Individually evaluated for impairment	0	0	0	0
Loans at period end, Collectively evaluated for impairment	5,450,705	4,733,764	5,450,705	4,733,764
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	262,887	373,586	262,887	373,586
Consumer and other
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	1,276	878	1,528	1,242
Other adjustments	0	0	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(389)	(116)	(720)	(342)
Recoveries	72	52	143	139
Provision for credit losses	286	241	294	16
Allowance for loan losses at period end	1,245	1,055	1,245	1,055
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	1,245	1,055	1,245	1,055
Allowance for credit losses at period end, Individually evaluated for impairment	192	30	192	30
Allowance for credit losses at period end, Collectively evaluated for impairment	1,053	1,024	1,053	1,024
Loans at period end, Individually evaluated for impairment	602	430	602	430
Loans at period end, Collectively evaluated for impairment	119,162	131,472	119,162	131,472
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	1	0	1
Loans at period end, Loans acquired with deteriorated credit quality	$ 1,211	$ 0	$ 1,211	$ 0