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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	$ 160,421	$ 143,402	$ 152,770	$ 137,905
Other adjustments	(13)	(18)	(51)	(102)
Reclassification from allowance for unfunded lending-related commitments	(30)	(2)	(116)	24
Charge-offs	(11,186)	(6,981)	(41,654)	(23,129)
Recoveries	1,737	2,313	4,776	10,627
Provision for credit losses	10,834	11,042	46,038	24,431
Allowance for loan losses at period end	161,763	149,756	161,763	149,756
Allowance for unfunded lending-related commitments at period end	1,510	1,245	1,510	1,245
Allowance for credit losses at period end	163,273	151,001	163,273	151,001
Individually evaluated for impairment	14,940	14,375	14,940	14,375
Collectively evaluated for impairment	147,860	135,942	147,860	135,942
Loans at period end, Individually evaluated for impairment	131,050	132,799	131,050	132,799
Loans at period end, Collectively evaluated for impairment	25,177,090	22,608,543	25,177,090	22,608,543
Loans at period end, Loans acquired with deteriorated credit quality	286,009	304,726	286,009	304,726
Loans held at fair value	116,022	77,883	116,022	77,883
Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at period end	473	684	473	684
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	74,893	60,727	67,826	57,811
Other adjustments	0	(1)	0	(3)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(6,775)	(3,219)	(24,658)	(8,116)
Recoveries	367	304	974	1,232
Provision for credit losses	4,642	8,934	28,985	15,821
Allowance for loan losses at period end	73,127	66,745	73,127	66,745
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	73,127	66,745	73,127	66,745
Individually evaluated for impairment	10,606	10,164	10,606	10,164
Collectively evaluated for impairment	62,160	55,987	62,160	55,987
Loans at period end, Individually evaluated for impairment	58,030	67,381	58,030	67,381
Loans at period end, Collectively evaluated for impairment	8,122,040	7,390,781	8,122,040	7,390,781
Loans at period end, Loans acquired with deteriorated credit quality	15,532	15,796	15,532	15,796
Loans held at fair value	0	0	0	0
Commercial | Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at period end	361	594	361	594
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	63,270	57,660	60,267	55,227
Other adjustments	0	(15)	(35)	(66)
Reclassification from allowance for unfunded lending-related commitments	(30)	(2)	(116)	24
Charge-offs	(809)	(208)	(4,869)	(1,176)
Recoveries	385	193	1,112	4,267
Provision for credit losses	2,122	619	8,579	(29)
Allowance for loan losses at period end	64,938	58,247	64,938	58,247
Allowance for unfunded lending-related commitments at period end	1,510	1,245	1,510	1,245
Allowance for credit losses at period end	66,448	59,492	66,448	59,492
Individually evaluated for impairment	3,600	3,158	3,600	3,158
Collectively evaluated for impairment	62,795	56,316	62,795	56,316
Loans at period end, Individually evaluated for impairment	32,155	31,952	32,155	31,952
Loans at period end, Collectively evaluated for impairment	7,299,229	6,610,140	7,299,229	6,610,140
Loans at period end, Loans acquired with deteriorated credit quality	117,283	104,682	117,283	104,682
Loans held at fair value	0	0	0	0
Commercial real estate | Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at period end	53	18	53	18
Home equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	3,631	9,551	8,507	10,493
Other adjustments	0	(2)	(20)	(2)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(1,594)	(561)	(2,372)	(1,530)
Recoveries	183	142	313	436
Provision for credit losses	1,500	13	(2,708)	(254)
Allowance for loan losses at period end	3,720	9,143	3,720	9,143
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	3,720	9,143	3,720	9,143
Individually evaluated for impairment	313	611	313	611
Collectively evaluated for impairment	3,407	8,532	3,407	8,532
Loans at period end, Individually evaluated for impairment	18,702	11,284	18,702	11,284
Loans at period end, Collectively evaluated for impairment	493,601	567,560	493,601	567,560
Loans at period end, Loans acquired with deteriorated credit quality	0	0	0	0
Loans held at fair value	0	0	0	0
Home equity | Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at period end	0	0	0	0
Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	8,146	6,336	7,194	6,688
Other adjustments	0	(14)	(15)	(19)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(25)	(337)	(315)	(1,088)
Recoveries	203	466	372	2,028
Provision for credit losses	1,087	(160)	2,175	(1,318)
Allowance for loan losses at period end	9,411	6,291	9,411	6,291
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	9,411	6,291	9,411	6,291
Individually evaluated for impairment	315	325	315	325
Collectively evaluated for impairment	9,037	5,894	9,037	5,894
Loans at period end, Individually evaluated for impairment	21,889	21,781	21,889	21,781
Loans at period end, Collectively evaluated for impairment	1,070,795	815,442	1,070,795	815,442
Loans at period end, Loans acquired with deteriorated credit quality	9,960	9,144	9,960	9,144
Loans held at fair value	116,022	77,883	116,022	77,883
Residential real estate | Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at period end	59	72	59	72
Premium finance receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	8,940	7,734	7,715	6,846
Other adjustments	(13)	14	19	(12)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(1,866)	(2,512)	(9,085)	(10,487)
Recoveries	563	1,142	1,853	2,502
Provision for credit losses	1,742	1,796	8,864	9,325
Allowance for loan losses at period end	9,366	8,174	9,366	8,174
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	9,366	8,174	9,366	8,174
Individually evaluated for impairment	0	0	0	0
Collectively evaluated for impairment	9,366	8,174	9,366	8,174
Loans at period end, Individually evaluated for impairment	0	0	0	0
Loans at period end, Collectively evaluated for impairment	8,104,538	7,110,808	8,104,538	7,110,808
Loans at period end, Loans acquired with deteriorated credit quality	140,908	173,490	140,908	173,490
Loans held at fair value	0	0	0	0
Premium finance receivables | Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at period end	0	0	0	0
Consumer and other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	1,541	1,394	1,261	840
Other adjustments	0	0	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(117)	(144)	(355)	(732)
Recoveries	36	66	152	162
Provision for credit losses	(259)	(160)	143	886
Allowance for loan losses at period end	1,201	1,156	1,201	1,156
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	1,201	1,156	1,201	1,156
Individually evaluated for impairment	106	117	106	117
Collectively evaluated for impairment	1,095	1,039	1,095	1,039
Loans at period end, Individually evaluated for impairment	274	401	274	401
Loans at period end, Collectively evaluated for impairment	86,887	113,812	86,887	113,812
Loans at period end, Loans acquired with deteriorated credit quality	2,326	1,614	2,326	1,614
Loans held at fair value	0	0	0	0
Consumer and other | Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at period end	$ 0	$ 0	$ 0	$ 0