Allowance for Credit Losses (Summary of Activity in the Allowance for Credit Losses) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	$ 373,109	$ 161,901	$ 158,461	$ 154,164
Other adjustments	55	(13)	24	(51)
Charge-offs	(11,763)	(11,186)	(36,175)	(41,654)
Recoveries	2,479	1,737	6,203	4,776
Provision for credit losses	25,028	10,834	213,051	46,038
Allowance for credit losses at period end	388,908	163,273	388,908	163,273
Individually measured	16,367	14,940	16,367	14,940
Collectively evaluated for impairment	372,541	147,860	372,541	147,860
Loans at period end, Individually evaluated for impairment	183,667	131,050	183,667	131,050
Loans at period end, Collectively evaluated for impairment	31,698,373	25,177,090	31,698,373	25,177,090
Loans at period end, Loans acquired with deteriorated credit quality	31,809,596	25,548,408	31,809,596	25,548,408	$ 26,643,462
Loans held at fair value	253,515	116,022	253,515	116,022
Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at period end		473		473
Loans at period end, Loans acquired with deteriorated credit quality		286,009		286,009
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	133,597	74,893	64,920	67,826
Other adjustments	0	0	0	0
Charge-offs	(5,270)	(6,775)	(13,109)	(24,658)
Recoveries	428	367	924	974
Provision for credit losses	(15,861)	4,642	51,120	28,985
Allowance for credit losses at period end	112,894	73,127	112,894	73,127
Individually measured	10,642	10,606	10,642	10,606
Collectively evaluated for impairment	102,252	62,160	102,252	62,160
Loans at period end, Individually evaluated for impairment	49,900	58,030	49,900	58,030
Loans at period end, Collectively evaluated for impairment	12,227,099	8,122,040	12,227,099	8,122,040
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		361
Loans at period end, Loans acquired with deteriorated credit quality		15,532		15,532
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	197,126	64,750	68,511	61,661
Other adjustments	0	0	0	(35)
Charge-offs	(1,529)	(809)	(9,323)	(4,869)
Recoveries	175	385	931	1,112
Provision for credit losses	38,238	2,122	141,827	8,579
Allowance for credit losses at period end	234,010	66,448	234,010	66,448
Individually measured	4,741	3,600	4,741	3,600
Collectively evaluated for impairment	229,269	62,795	229,269	62,795
Loans at period end, Individually evaluated for impairment	79,661	32,155	79,661	32,155
Loans at period end, Collectively evaluated for impairment	8,343,481	7,299,229	8,343,481	7,299,229
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		53
Loans at period end, Loans acquired with deteriorated credit quality		117,283		117,283
Home equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	12,189	3,631	3,878	8,507
Other adjustments	0	0	0	(20)
Charge-offs	(138)	(1,594)	(1,378)	(2,372)
Recoveries	111	183	451	313
Provision for credit losses	(485)	1,500	(335)	(2,708)
Allowance for credit losses at period end	11,677	3,720	11,677	3,720
Individually measured	196	313	196	313
Collectively evaluated for impairment	11,481	3,407	11,481	3,407
Loans at period end, Individually evaluated for impairment	22,962	18,702	22,962	18,702
Loans at period end, Collectively evaluated for impairment	423,312	493,601	423,312	493,601
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
Loans at period end, Loans acquired with deteriorated credit quality		0		0
Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	11,915	8,146	9,800	7,194
Other adjustments	0	0	0	(15)
Charge-offs	(83)	(25)	(777)	(315)
Recoveries	25	203	115	372
Provision for credit losses	(67)	1,087	(350)	2,175
Allowance for credit losses at period end	11,790	9,411	11,790	9,411
Individually measured	680	315	680	315
Collectively evaluated for impairment	11,110	9,037	11,110	9,037
Loans at period end, Individually evaluated for impairment	30,596	21,889	30,596	21,889
Loans at period end, Collectively evaluated for impairment	1,100,699	1,070,795	1,100,699	1,070,795
Loans held at fair value	253,515	116,022	253,515	116,022
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		59
Loans at period end, Loans acquired with deteriorated credit quality		9,960		9,960
Premium finance receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	17,592	8,940	9,647	7,715
Other adjustments	55	(13)	24	19
Charge-offs	(4,640)	(1,866)	(11,258)	(9,085)
Recoveries	1,720	563	3,663	1,853
Provision for credit losses	3,129	1,742	20,739	8,864
Allowance for credit losses at period end	17,856	9,366	17,856	9,366
Individually measured	0	0	0	0
Collectively evaluated for impairment	17,856	9,366	17,856	9,366
Loans at period end, Individually evaluated for impairment	0	0	0	0
Loans at period end, Collectively evaluated for impairment	9,548,976	8,104,538	9,548,976	8,104,538
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
Loans at period end, Loans acquired with deteriorated credit quality		140,908		140,908
Consumer and other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	690	1,541	1,705	1,261
Other adjustments	0	0	0	0
Charge-offs	(103)	(117)	(330)	(355)
Recoveries	20	36	119	152
Provision for credit losses	74	(259)	50	143
Allowance for credit losses at period end	681	1,201	681	1,201
Individually measured	108	106	108	106
Collectively evaluated for impairment	573	1,095	573	1,095
Loans at period end, Individually evaluated for impairment	548	274	548	274
Loans at period end, Collectively evaluated for impairment	54,806	86,887	54,806	86,887
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
Loans at period end, Loans acquired with deteriorated credit quality		$ 2,326		$ 2,326
Cumulative effect adjustment from the adoption of ASU 2016-13
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			47,344
Cumulative effect adjustment from the adoption of ASU 2016-13 | Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			9,039
Cumulative effect adjustment from the adoption of ASU 2016-13 | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			32,064
Cumulative effect adjustment from the adoption of ASU 2016-13 | Home equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			9,061
Cumulative effect adjustment from the adoption of ASU 2016-13 | Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			3,002
Cumulative effect adjustment from the adoption of ASU 2016-13 | Premium finance receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			(4,959)
Cumulative effect adjustment from the adoption of ASU 2016-13 | Consumer and other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period			$ (863)