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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	$ 304,031	$ 373,109	$ 379,910	$ 158,461
Other adjustments	(65)	55	(2)	24
Charge-offs	(3,329)	(11,763)	(26,420)	(36,175)
Recoveries	3,327	2,479	11,151	6,203
Provision for credit losses	(7,909)	25,028	(68,584)	213,051
Allowance for credit losses at period end	296,055	388,908	296,055	388,908
Individually measured	10,759	16,367	10,759	16,367
Collectively measured	285,296	372,541	285,296	372,541
Loans at period end, individually evaluated for impairment	109,011	183,667	109,011	183,667
Loans at period end, collectively evaluated for impairment	33,124,053	31,698,373	33,124,053	31,698,373
Loans held at fair value	30,979	253,515	30,979	253,515
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	98,507	133,597	94,212	64,920
Other adjustments		0	0	0
Charge-offs	(1,352)	(5,270)	(16,370)	(13,109)
Recoveries	816	428	2,170	924
Provision for credit losses	11,811	(15,861)	29,770	51,120
Allowance for credit losses at period end	109,782	112,894	109,782	112,894
Individually measured	8,222	10,642	8,222	10,642
Collectively measured	101,560	102,252	101,560	102,252
Loans at period end, individually evaluated for impairment	31,001	49,900	31,001	49,900
Loans at period end, collectively evaluated for impairment	11,156,971	12,227,099	11,156,971	12,227,099
Loans held at fair value	0	0	0	0
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	158,522	197,126	243,603	68,511
Other adjustments	0	0	0	0
Charge-offs	(406)	(1,529)	(2,798)	(9,323)
Recoveries	373	175	1,087	931
Provision for credit losses	(18,454)	38,238	(101,857)	141,827
Allowance for credit losses at period end	140,035	234,010	140,035	234,010
Individually measured	1,308	4,741	1,308	4,741
Collectively measured	138,727	229,269	138,727	229,269
Loans at period end, individually evaluated for impairment	32,091	79,661	32,091	79,661
Loans at period end, collectively evaluated for impairment	8,853,623	8,343,481	8,853,623	8,343,481
Loans held at fair value	0	0	0	0
Home equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	11,207	12,189	11,437	3,878
Other adjustments	0	0	0	0
Charge-offs	(59)	(138)	(201)	(1,378)
Recoveries	313	111	742	451
Provision for credit losses	(522)	(485)	(1,039)	(335)
Allowance for credit losses at period end	10,939	11,677	10,939	11,677
Individually measured	221	196	221	196
Collectively measured	10,718	11,481	10,718	11,481
Loans at period end, individually evaluated for impairment	16,486	22,962	16,486	22,962
Loans at period end, collectively evaluated for impairment	331,176	423,312	331,176	423,312
Loans held at fair value	0	0	0	0
Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	15,684	11,915	12,459	9,800
Other adjustments	0	0	0	0
Charge-offs	(10)	(83)	(15)	(777)
Recoveries	5	25	245	115
Provision for credit losses	593	(67)	3,583	(350)
Allowance for credit losses at period end	16,272	11,790	16,272	11,790
Individually measured	940	680	940	680
Collectively measured	15,332	11,110	15,332	11,110
Loans at period end, individually evaluated for impairment	28,966	30,596	28,966	30,596
Loans at period end, collectively evaluated for impairment	1,487,791	1,100,699	1,487,791	1,100,699
Loans held at fair value	30,979	253,515	30,979	253,515
Premium finance receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	19,899	17,592	17,777	9,647
Other adjustments	(65)	55	(2)	24
Charge-offs	(1,390)	(4,640)	(6,706)	(11,258)
Recoveries	1,728	1,720	6,749	3,663
Provision for credit losses	(1,597)	3,129	757	20,739
Allowance for credit losses at period end	18,575	17,856	18,575	17,856
Individually measured	0	0	0	0
Collectively measured	18,575	17,856	18,575	17,856
Loans at period end, individually evaluated for impairment	0	0	0	0
Loans at period end, collectively evaluated for impairment	11,272,430	9,548,976	11,272,430	9,548,976
Loans held at fair value	0	0	0	0
Consumer and other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	212	690	422	1,705
Other adjustments	0	0	0	0
Charge-offs	(112)	(103)	(330)	(330)
Recoveries	92	20	158	119
Provision for credit losses	260	74	202	50
Allowance for credit losses at period end	452	681	452	681
Individually measured	68	108	68	108
Collectively measured	384	573	384	573
Loans at period end, individually evaluated for impairment	467	548	467	548
Loans at period end, collectively evaluated for impairment	22,062	54,806	22,062	54,806
Loans held at fair value	$ 0	$ 0	$ 0	0
Cumulative effect, period of adoption, adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period				47,344
Cumulative effect, period of adoption, adjustment | Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period				9,039
Cumulative effect, period of adoption, adjustment | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period				32,064
Cumulative effect, period of adoption, adjustment | Home equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period				9,061
Cumulative effect, period of adoption, adjustment | Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period				3,002
Cumulative effect, period of adoption, adjustment | Premium finance receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period				(4,959)
Cumulative effect, period of adoption, adjustment | Consumer and other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period				$ (863)