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, 2018	Mar. 31, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	$ 137,905	$ 122,291
Other adjustments	(40)	(56)
Reclassification from allowance for unfunded lending-related commitments	26	(138)
Charge-offs	(9,278)	(3,417)
Recoveries	2,544	1,823
Provision for credit losses	8,346	5,316
Allowance for loan losses at period end	139,503	125,819
Allowance for unfunded lending-related commitments at period end	1,243	1,811
Allowance for credit losses at period end	140,746	127,630
Individually evaluated for impairment	6,877	8,780
Collectively evaluated for impairment	133,194	118,085
Loans at period end, Individually evaluated for impairment	103,455	88,303
Loans at period end, Collectively evaluated for impairment	21,564,972	19,399,837
Loans held at fair value	67,962	28,548
Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	675	765
Loans at period end, Loans acquired with deteriorated credit quality	325,745	414,370
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	57,811	44,493
Other adjustments	(1)	(19)
Reclassification from allowance for unfunded lending-related commitments	0	(92)
Charge-offs	(2,687)	(641)
Recoveries	262	273
Provision for credit losses	2,251	2,568
Allowance for loan losses at period end	57,636	46,582
Allowance for unfunded lending-related commitments at period end	0	592
Allowance for credit losses at period end	57,636	47,174
Individually evaluated for impairment	2,344	2,845
Collectively evaluated for impairment	54,789	43,687
Loans at period end, Individually evaluated for impairment	33,810	19,319
Loans at period end, Collectively evaluated for impairment	7,017,831	6,050,684
Loans held at fair value	0	0
Commercial | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	503	642
Loans at period end, Loans acquired with deteriorated credit quality	9,230	11,486
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	55,227	51,422
Other adjustments	(24)	(36)
Reclassification from allowance for unfunded lending-related commitments	26	(46)
Charge-offs	(813)	(261)
Recoveries	1,687	554
Provision for credit losses	1,378	1,000
Allowance for loan losses at period end	57,481	52,633
Allowance for unfunded lending-related commitments at period end	1,243	1,219
Allowance for credit losses at period end	58,724	53,852
Individually evaluated for impairment	3,611	3,198
Collectively evaluated for impairment	55,042	50,594
Loans at period end, Individually evaluated for impairment	38,237	40,107
Loans at period end, Collectively evaluated for impairment	6,479,737	6,064,239
Loans held at fair value	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	71	60
Loans at period end, Loans acquired with deteriorated credit quality	115,546	157,336
Home equity
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	10,493	11,774
Other adjustments	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(357)	(625)
Recoveries	123	65
Provision for credit losses	(399)	989
Allowance for loan losses at period end	9,860	12,203
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	9,860	12,203
Individually evaluated for impairment	749	1,979
Collectively evaluated for impairment	9,111	10,224
Loans at period end, Individually evaluated for impairment	10,102	11,878
Loans at period end, Collectively evaluated for impairment	616,445	696,380
Loans held at fair value	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
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	6,688	5,714
Other adjustments	(3)	(4)
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(571)	(329)
Recoveries	40	178
Provision for credit losses	124	(29)
Allowance for loan losses at period end	6,278	5,530
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	6,278	5,530
Individually evaluated for impairment	148	666
Collectively evaluated for impairment	6,029	4,802
Loans at period end, Individually evaluated for impairment	20,558	16,594
Loans at period end, Collectively evaluated for impairment	768,859	671,765
Loans held at fair value	67,962	28,548
Residential real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	101	62
Loans at period end, Loans acquired with deteriorated credit quality	11,725	3,701
Premium finance receivables
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	6,846	7,625
Other adjustments	(12)	3
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(4,721)	(1,427)
Recoveries	385	612
Provision for credit losses	4,835	746
Allowance for loan losses at period end	7,333	7,559
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	7,333	7,559
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	7,333	7,559
Loans at period end, Individually evaluated for impairment	0	0
Loans at period end, Collectively evaluated for impairment	6,578,876	5,799,803
Loans held at fair value	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
Loans at period end, Loans acquired with deteriorated credit quality	187,235	240,706
Consumer and other
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	840	1,263
Other adjustments	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(129)	(134)
Recoveries	47	141
Provision for credit losses	157	42
Allowance for loan losses at period end	915	1,312
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	915	1,312
Individually evaluated for impairment	25	92
Collectively evaluated for impairment	890	1,219
Loans at period end, Individually evaluated for impairment	748	405
Loans at period end, Collectively evaluated for impairment	103,224	116,966
Loans held at fair value	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	1
Loans at period end, Loans acquired with deteriorated credit quality	$ 2,009	$ 1,141