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
	Mar. 31, 2019	Mar. 31, 2018
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	$ 152,770	$ 137,905
Other adjustments	(27)	(40)
Reclassification from allowance for unfunded lending-related commitments	(16)	26
Charge-offs	(6,640)	(9,278)
Recoveries	1,501	2,544
Provision for credit losses	10,624	8,346
Allowance for loan losses at period end	158,212	139,503
Allowance for unfunded lending-related commitments at period end	1,410	1,243
Allowance for credit losses at period end	159,622	140,746
Individually evaluated for impairment	13,606	6,877
Collectively evaluated for impairment	145,372	133,194
Loans at period end, Individually evaluated for impairment	144,361	103,455
Loans at period end, Collectively evaluated for impairment	23,655,976	21,564,972
Loans held at fair value	101,071	67,962
Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	644	675
Loans at period end, Loans acquired with deteriorated credit quality	313,221	325,745
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	67,826	57,811
Other adjustments	0	(1)
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(503)	(2,687)
Recoveries	318	262
Provision for credit losses	6,997	2,251
Allowance for loan losses at period end	74,638	57,636
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	74,638	57,636
Individually evaluated for impairment	11,858	2,344
Collectively evaluated for impairment	62,317	54,789
Loans at period end, Individually evaluated for impairment	75,442	33,810
Loans at period end, Collectively evaluated for impairment	7,893,419	7,017,831
Loans held at fair value	0	0
Commercial | Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	463	503
Loans at period end, Loans acquired with deteriorated credit quality	25,330	9,230
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	60,267	55,227
Other adjustments	(24)	(24)
Reclassification from allowance for unfunded lending-related commitments	(16)	26
Charge-offs	(3,734)	(813)
Recoveries	480	1,687
Provision for credit losses	877	1,378
Allowance for loan losses at period end	57,850	57,481
Allowance for unfunded lending-related commitments at period end	1,410	1,243
Allowance for credit losses at period end	59,260	58,724
Individually evaluated for impairment	517	3,611
Collectively evaluated for impairment	58,623	55,042
Loans at period end, Individually evaluated for impairment	30,300	38,237
Loans at period end, Collectively evaluated for impairment	6,832,544	6,479,737
Loans held at fair value	0	0
Commercial real estate | Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	120	71
Loans at period end, Loans acquired with deteriorated credit quality	110,661	115,546
Home equity
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	8,507	10,493
Other adjustments	(7)	0
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(88)	(357)
Recoveries	62	123
Provision for credit losses	153	(399)
Allowance for loan losses at period end	8,627	9,860
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	8,627	9,860
Individually evaluated for impairment	796	749
Collectively evaluated for impairment	7,831	9,111
Loans at period end, Individually evaluated for impairment	15,779	10,102
Loans at period end, Collectively evaluated for impairment	512,669	616,445
Loans held at fair value	0	0
Home equity | Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [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 Losses [Roll Forward]
Allowance for loan losses at beginning of period	7,194	6,688
Other adjustments	(7)	(3)
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(3)	(571)
Recoveries	29	40
Provision for credit losses	417	124
Allowance for loan losses at period end	7,630	6,278
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	7,630	6,278
Individually evaluated for impairment	302	148
Collectively evaluated for impairment	7,267	6,029
Loans at period end, Individually evaluated for impairment	22,464	20,558
Loans at period end, Collectively evaluated for impairment	921,204	768,859
Loans held at fair value	101,071	67,962
Residential real estate | Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	61	101
Loans at period end, Loans acquired with deteriorated credit quality	8,785	11,725
Premium finance receivables
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	7,715	6,846
Other adjustments	11	(12)
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(2,210)	(4,721)
Recoveries	556	385
Provision for credit losses	2,147	4,835
Allowance for loan losses at period end	8,219	7,333
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	8,219	7,333
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	8,219	7,333
Loans at period end, Individually evaluated for impairment	0	0
Loans at period end, Collectively evaluated for impairment	7,378,387	6,578,876
Loans held at fair value	0	0
Premium finance receivables | Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	0	0
Loans at period end, Loans acquired with deteriorated credit quality	165,770	187,235
Consumer and other
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	1,261	840
Other adjustments	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0
Charge-offs	(102)	(129)
Recoveries	56	47
Provision for credit losses	33	157
Allowance for loan losses at period end	1,248	915
Allowance for unfunded lending-related commitments at period end	0	0
Allowance for credit losses at period end	1,248	915
Individually evaluated for impairment	133	25
Collectively evaluated for impairment	1,115	890
Loans at period end, Individually evaluated for impairment	376	748
Loans at period end, Collectively evaluated for impairment	117,753	103,224
Loans held at fair value	0	0
Consumer and other | Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	0	0
Loans at period end, Loans acquired with deteriorated credit quality	$ 2,675	$ 2,009