Allowance for Credit Losses (Summary of Activity in the Allowance for Credit Losses by Loan Portfolio) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	$ 299,653	$ 379,910
Other adjustments	22	30
Charge-offs	(4,725)	(16,116)
Recoveries	2,193	2,771
Provision for credit losses	4,025	(45,386)
Allowance for credit losses at period end	301,168	321,209
Individually measured	5,152	8,329
Collectively measured	296,016	312,880
Loans at period end, individually measured	72,846	124,404
Loans at period end, collectively measured	35,149,717	32,994,913
Loans held at fair value	57,984	51,916
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	119,307	94,212
Other adjustments	0	0
Charge-offs	(1,414)	(11,781)
Recoveries	538	452
Provision for credit losses	2,480	12,757
Allowance for credit losses at period end	120,911	95,640
Individually measured	3,698	5,046
Collectively measured	117,213	90,594
Loans at period end, individually measured	19,651	30,144
Loans at period end, collectively measured	11,564,312	12,678,063
Loans held at fair value	0	0
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	144,583	243,603
Other adjustments	0	0
Charge-offs	(777)	(980)
Recoveries	32	200
Provision for credit losses	1,068	(61,031)
Allowance for credit losses at period end	144,906	181,792
Individually measured	522	2,042
Collectively measured	144,384	179,750
Loans at period end, individually measured	22,370	43,858
Loans at period end, collectively measured	9,212,704	8,500,921
Loans held at fair value	0	0
Home equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	10,699	11,437
Other adjustments	0	0
Charge-offs	(197)	0
Recoveries	93	101
Provision for credit losses	(29)	(156)
Allowance for credit losses at period end	10,566	11,382
Individually measured	127	338
Collectively measured	10,439	11,044
Loans at period end, individually measured	12,904	21,167
Loans at period end, collectively measured	308,531	369,086
Loans held at fair value	0	0
Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	8,782	12,459
Other adjustments	0	0
Charge-offs	(466)	(2)
Recoveries	5	204
Provision for credit losses	1,108	1,581
Allowance for credit losses at period end	9,429	14,242
Individually measured	800	822
Collectively measured	8,629	13,420
Loans at period end, individually measured	17,842	28,675
Loans at period end, collectively measured	1,724,159	1,341,382
Loans held at fair value	57,984	51,916
Premium finance receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	15,859	17,777
Other adjustments	22	30
Charge-offs	(1,678)	(3,239)
Recoveries	1,476	1,782
Provision for credit losses	(957)	1,127
Allowance for credit losses at period end	14,722	17,477
Individually measured	0	0
Collectively measured	14,722	17,477
Loans at period end, individually measured	0	0
Loans at period end, collectively measured	12,291,571	10,070,038
Loans held at fair value	0	0
Consumer and other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	423	422
Other adjustments	0	0
Charge-offs	(193)	(114)
Recoveries	49	32
Provision for credit losses	355	336
Allowance for credit losses at period end	634	676
Individually measured	5	81
Collectively measured	629	595
Loans at period end, individually measured	79	560
Loans at period end, collectively measured	48,440	35,423
Loans held at fair value	$ 0	$ 0