Allowance for Credit Losses (Summary of Activity in the Allowance for Credit Losses) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	$ 253,412	$ 159,622	$ 158,461	$ 154,164
Other adjustments	42	(11)	(31)	(38)
Charge-offs	(16,975)	(23,828)	(24,412)	(30,468)
Recoveries	1,572	1,538	3,724	3,039
Provision for credit losses	135,058	24,580	188,023	35,204
Allowance for credit losses at period end	373,109	161,901	373,109	161,901
Individually measured	18,483	15,206	18,483	15,206
Collectively evaluated for impairment	354,626	146,164	354,626	146,164
Loans at period end, Individually evaluated for impairment	183,365	137,514	183,365	137,514
Loans at period end, Collectively evaluated for impairment	30,964,924	24,750,279	30,964,924	24,750,279
Loans held at fair value	254,614	106,081	254,614	106,081
Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at period end		531		531
Loans at period end, Loans acquired with deteriorated credit quality		310,785		310,785
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	107,346	74,638	64,920	67,826
Other adjustments	0	0	0	0
Charge-offs	(5,686)	(17,380)	(7,839)	(17,883)
Recoveries	112	289	495	607
Provision for credit losses	31,825	17,346	66,982	24,343
Allowance for credit losses at period end	133,597	74,893	133,597	74,893
Individually measured	12,689	10,588	12,689	10,588
Collectively evaluated for impairment	120,908	63,891	120,908	63,891
Loans at period end, Individually evaluated for impairment	48,220	63,528	48,220	63,528
Loans at period end, Collectively evaluated for impairment	11,811,012	8,182,921	11,811,012	8,182,921
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		414		414
Loans at period end, Loans acquired with deteriorated credit quality		24,325		24,325
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	112,796	59,260	68,511	61,661
Other adjustments	0	(11)	0	(35)
Charge-offs	(7,224)	(326)	(7,794)	(4,060)
Recoveries	493	247	756	727
Provision for credit losses	91,061	5,580	103,589	6,457
Allowance for credit losses at period end	197,126	64,750	197,126	64,750
Individually measured	5,023	3,979	5,023	3,979
Collectively evaluated for impairment	192,103	60,717	192,103	60,717
Loans at period end, Individually evaluated for impairment	83,664	33,749	83,664	33,749
Loans at period end, Collectively evaluated for impairment	8,117,081	7,115,504	8,117,081	7,115,504
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		54		54
Loans at period end, Loans acquired with deteriorated credit quality		126,991		126,991
Home equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	12,394	8,627	3,878	8,507
Other adjustments	0	(13)	0	(20)
Charge-offs	(239)	(690)	(1,240)	(778)
Recoveries	46	68	340	130
Provision for credit losses	(12)	(4,361)	150	(4,208)
Allowance for credit losses at period end	12,189	3,631	12,189	3,631
Individually measured	264	209	264	209
Collectively evaluated for impairment	11,925	3,422	11,925	3,422
Loans at period end, Individually evaluated for impairment	22,782	18,303	22,782	18,303
Loans at period end, Collectively evaluated for impairment	443,814	509,067	443,814	509,067
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	12,550	7,630	9,800	7,194
Other adjustments	0	(8)	0	(15)
Charge-offs	(293)	(287)	(694)	(290)
Recoveries	30	140	90	169
Provision for credit losses	(372)	671	(283)	1,088
Allowance for credit losses at period end	11,915	8,146	11,915	8,146
Individually measured	393	321	393	321
Collectively evaluated for impairment	11,522	7,762	11,522	7,762
Loans at period end, Individually evaluated for impairment	28,145	21,663	28,145	21,663
Loans at period end, Collectively evaluated for impairment	1,144,670	980,167	1,144,670	980,167
Loans held at fair value	254,614	106,081	254,614	106,081
Residential real estate | Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at period end		63		63
Loans at period end, Loans acquired with deteriorated credit quality		10,267		10,267
Premium finance receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	7,880	8,219	9,647	7,715
Other adjustments	42	21	(31)	32
Charge-offs	(3,434)	(5,009)	(6,618)	(7,219)
Recoveries	833	734	1,943	1,290
Provision for credit losses	12,271	4,975	17,610	7,122
Allowance for credit losses at period end	17,592	8,940	17,592	8,940
Individually measured	0	0	0	0
Collectively evaluated for impairment	17,592	8,940	17,592	8,940
Loans at period end, Individually evaluated for impairment	0	0	0	0
Loans at period end, Collectively evaluated for impairment	9,400,576	7,856,344	9,400,576	7,856,344
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		146,557		146,557
Consumer and other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	446	1,248	1,705	1,261
Other adjustments	0	0	0	0
Charge-offs	(99)	(136)	(227)	(238)
Recoveries	58	60	100	116
Provision for credit losses	285	369	(25)	402
Allowance for credit losses at period end	690	1,541	690	1,541
Individually measured	114	109	114	109
Collectively evaluated for impairment	576	1,432	576	1,432
Loans at period end, Individually evaluated for impairment	554	271	554	271
Loans at period end, Collectively evaluated for impairment	47,771	106,276	47,771	106,276
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		$ 2,645		$ 2,645
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)