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
	Mar. 31, 2016	Mar. 31, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	$ 105,400	$ 91,705
Other adjustments	(78)	(248)
Reclassification from allowance for unfunded lending-related commitments	(81)	(113)
Charge-offs	(5,474)	(4,271)
Recoveries	1,981	1,188
Provision for credit losses	8,423	6,185
Allowance for loan losses at period end	110,171	94,446
Allowance for unfunded lending-related commitments at period end	1,030	888
Allowance for credit losses at period end	111,201	95,334
Allowance for credit losses at period end, Individually evaluated for impairment	7,812	6,252
Allowance for credit losses at period end, Collectively evaluated for impairment	102,637	88,875
Loans at period end, Individually evaluated for impairment	96,997	113,651
Loans at period end, Collectively evaluated for impairment	16,866,468	14,371,978
Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	752	207
Loans at period end, Loans acquired with deteriorated credit quality	482,948	467,430
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	36,135	31,699
Other adjustments	(9)	(17)
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(671)	(677)
Recoveries	629	370
Provision for credit losses	2,351	2,351
Allowance for loan losses at period end	38,435	33,726
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	38,435	33,726
Allowance for credit losses at period end, Individually evaluated for impairment	2,319	1,814
Allowance for credit losses at period end, Collectively evaluated for impairment	35,448	31,912
Loans at period end, Individually evaluated for impairment	17,969	12,361
Loans at period end, Collectively evaluated for impairment	4,852,198	4,190,224
Commercial | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	668	0
Loans at period end, Loans acquired with deteriorated credit quality	20,079	9,347
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	43,758	35,533
Other adjustments	(76)	(180)
Reclassification from allowance for unfunded lending-related commitments	(81)	(113)
Charge-offs	(671)	(1,005)
Recoveries	369	312
Provision for credit losses	1,964	2,455
Allowance for loan losses at period end	45,263	37,002
Allowance for unfunded lending-related commitments at period end	1,030	888
Allowance for credit losses at period end	46,293	37,890
Allowance for credit losses at period end, Individually evaluated for impairment	3,028	3,256
Allowance for credit losses at period end, Collectively evaluated for impairment	43,261	34,521
Loans at period end, Individually evaluated for impairment	52,977	75,886
Loans at period end, Collectively evaluated for impairment	5,521,632	4,567,928
Commercial real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	4	113
Loans at period end, Loans acquired with deteriorated credit quality	163,350	66,672
Home equity
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	12,012	12,500
Other adjustments	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(1,052)	(584)
Recoveries	48	48
Provision for credit losses	1,907	700
Allowance for loan losses at period end	12,915	12,664
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	12,915	12,664
Allowance for credit losses at period end, Individually evaluated for impairment	1,695	948
Allowance for credit losses at period end, Collectively evaluated for impairment	11,220	11,716
Loans at period end, Individually evaluated for impairment	9,365	7,879
Loans at period end, Collectively evaluated for impairment	764,977	701,404
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
Loans at period end, Loans acquired with deteriorated credit quality	0	0
Residential real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	4,734	4,218
Other adjustments	(30)	(3)
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(493)	(631)
Recoveries	112	76
Provision for credit losses	841	436
Allowance for loan losses at period end	5,164	4,096
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	5,164	4,096
Allowance for credit losses at period end, Individually evaluated for impairment	700	208
Allowance for credit losses at period end, Collectively evaluated for impairment	4,384	3,794
Loans at period end, Individually evaluated for impairment	16,159	17,144
Loans at period end, Collectively evaluated for impairment	606,503	476,418
Residential real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	80	94
Loans at period end, Loans acquired with deteriorated credit quality	3,381	2,363
Premium finance receivables
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	7,233	6,513
Other adjustments	37	(48)
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(2,480)	(1,263)
Recoveries	787	329
Provision for credit losses	1,628	461
Allowance for loan losses at period end	7,205	5,992
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	7,205	5,992
Allowance for credit losses at period end, Individually evaluated for impairment	0	0
Allowance for credit losses at period end, Collectively evaluated for impairment	7,205	5,992
Loans at period end, Individually evaluated for impairment	0	0
Loans at period end, Collectively evaluated for impairment	5,001,783	4,306,229
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
Loans at period end, Loans acquired with deteriorated credit quality	296,138	389,048
Consumer and other
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	1,528	1,242
Other adjustments	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(107)	(111)
Recoveries	36	53
Provision for credit losses	(268)	(218)
Allowance for loan losses at period end	1,189	966
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	1,189	966
Allowance for credit losses at period end, Individually evaluated for impairment	70	26
Allowance for credit losses at period end, Collectively evaluated for impairment	1,119	940
Loans at period end, Individually evaluated for impairment	527	381
Loans at period end, Collectively evaluated for impairment	119,375	129,775
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
Loans at period end, Loans acquired with deteriorated credit quality	$ 0	$ 0