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, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	$ 91,705	$ 96,922
Other adjustments	(248)	(148)
Reclassification from allowance for unfunded lending-related commitments	(113)	(18)
Charge-offs	(4,271)	(9,017)
Recoveries	1,188	1,232
Provision for credit losses	6,185	3,304
Allowance for loan losses at period end	94,446	92,275
Allowance for unfunded lending-related commitments at period end	888	737
Allowance for credit losses at period end	95,334	93,012
Allowance for credit losses at period end, Individually evaluated for impairment	6,252	8,294
Allowance for credit losses at period end, Collectively evaluated for impairment	88,875	84,559
Loans at period end, Individually evaluated for impairment	113,651	144,985
Loans at period end, Collectively evaluated for impairment	14,371,978	12,485,216
Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	207	159
Loans at period end, Loans acquired with deteriorated credit quality	467,430	502,959
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	31,699	23,092
Other adjustments	(17)	(15)
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(677)	(648)
Recoveries	370	317
Provision for credit losses	2,351	1,943
Allowance for loan losses at period end	33,726	24,689
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	33,726	24,689
Allowance for credit losses at period end, Individually evaluated for impairment	1,814	3,107
Allowance for credit losses at period end, Collectively evaluated for impairment	31,912	21,512
Loans at period end, Individually evaluated for impairment	12,361	18,350
Loans at period end, Collectively evaluated for impairment	4,190,224	3,410,011
Commercial | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	0	70
Loans at period end, Loans acquired with deteriorated credit quality	9,347	10,836
Commercial real-estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	35,533	48,658
Other adjustments	(180)	(121)
Reclassification from allowance for unfunded lending-related commitments	(113)	(18)
Charge-offs	(1,005)	(4,493)
Recoveries	312	145
Provision for credit losses	2,455	434
Allowance for loan losses at period end	37,002	44,605
Allowance for unfunded lending-related commitments at period end	888	737
Allowance for credit losses at period end	37,890	45,342
Allowance for credit losses at period end, Individually evaluated for impairment	3,256	4,041
Allowance for credit losses at period end, Collectively evaluated for impairment	34,521	41,301
Loans at period end, Individually evaluated for impairment	75,886	99,480
Loans at period end, Collectively evaluated for impairment	4,567,928	4,087,171
Commercial real-estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	113	0
Loans at period end, Loans acquired with deteriorated credit quality	66,672	75,604
Home equity
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	12,500	12,611
Other adjustments	0	(1)
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(584)	(2,267)
Recoveries	48	257
Provision for credit losses	700	366
Allowance for loan losses at period end	12,664	10,966
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	12,664	10,966
Allowance for credit losses at period end, Individually evaluated for impairment	948	596
Allowance for credit losses at period end, Collectively evaluated for impairment	11,716	10,370
Loans at period end, Individually evaluated for impairment	7,879	7,537
Loans at period end, Collectively evaluated for impairment	701,404	700,211
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	4,218	5,108
Other adjustments	(3)	(2)
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(631)	(226)
Recoveries	76	131
Provision for credit losses	436	(320)
Allowance for loan losses at period end	4,096	4,691
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	4,096	4,691
Allowance for credit losses at period end, Individually evaluated for impairment	208	455
Allowance for credit losses at period end, Collectively evaluated for impairment	3,794	4,147
Loans at period end, Individually evaluated for impairment	17,144	18,026
Loans at period end, Collectively evaluated for impairment	476,418	405,668
Residential real-estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	94	89
Loans at period end, Loans acquired with deteriorated credit quality	2,363	3,075
Premium finance receivables
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	6,513	5,583
Other adjustments	(48)	(9)
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(1,263)	(1,210)
Recoveries	329	321
Provision for credit losses	461	897
Allowance for loan losses at period end	5,992	5,582
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	5,992	5,582
Allowance for credit losses at period end, Individually evaluated for impairment	0	0
Allowance for credit losses at period end, Collectively evaluated for impairment	5,992	5,582
Loans at period end, Individually evaluated for impairment	0	0
Loans at period end, Collectively evaluated for impairment	4,306,229	3,724,493
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	389,048	413,202
Consumer and other
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	1,242	1,870
Other adjustments	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(111)	(173)
Recoveries	53	61
Provision for credit losses	(218)	(16)
Allowance for loan losses at period end	966	1,742
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	966	1,742
Allowance for credit losses at period end, Individually evaluated for impairment	26	95
Allowance for credit losses at period end, Collectively evaluated for impairment	940	1,647
Loans at period end, Individually evaluated for impairment	381	1,592
Loans at period end, Collectively evaluated for impairment	129,775	157,662
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	$ 242