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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	$ 158,212	$ 139,503	$ 152,770	$ 137,905
Other adjustments	(11)	(44)	(38)	(84)
Reclassification from allowance for unfunded lending-related commitments	(70)	0	(86)	26
Charge-offs	(23,828)	(6,870)	(30,468)	(16,148)
Recoveries	1,538	5,770	3,039	8,314
Provision for credit losses	24,580	5,043	35,204	13,389
Allowance for loan losses at period end	160,421	143,402	160,421	143,402
Allowance for unfunded lending-related commitments at period end	1,480	1,243	1,480	1,243
Allowance for credit losses at period end	161,901	144,645	161,901	144,645
Individually evaluated for impairment	15,206	8,554	15,206	8,554
Collectively evaluated for impairment	146,164	135,471	146,164	135,471
Loans at period end, Individually evaluated for impairment	137,514	120,610	137,514	120,610
Loans at period end, Collectively evaluated for impairment	24,750,279	22,111,452	24,750,279	22,111,452
Loans held at fair value	106,081	67,181	106,081	67,181
Loans acquired with deteriorated credit quality
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses at period end	531	620	531	620
Loans at period end, Loans acquired with deteriorated credit quality	310,785	311,317	310,785	311,317
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	74,638	57,636	67,826	57,811
Other adjustments	0	(1)	0	(2)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(17,380)	(2,210)	(17,883)	(4,897)
Recoveries	289	666	607	928
Provision for credit losses	17,346	4,636	24,343	6,887
Allowance for loan losses at period end	74,893	60,727	74,893	60,727
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	74,893	60,727	74,893	60,727
Individually evaluated for impairment	10,588	4,406	10,588	4,406
Collectively evaluated for impairment	63,891	55,834	63,891	55,834
Loans at period end, Individually evaluated for impairment	63,528	55,948	63,528	55,948
Loans at period end, Collectively evaluated for impairment	8,182,921	7,224,562	8,182,921	7,224,562
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	487	414	487
Loans at period end, Loans acquired with deteriorated credit quality	24,325	8,550	24,325	8,550
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	57,850	57,481	60,267	55,227
Other adjustments	(11)	(27)	(35)	(51)
Reclassification from allowance for unfunded lending-related commitments	(70)	0	(86)	26
Charge-offs	(326)	(155)	(4,060)	(968)
Recoveries	247	2,387	727	4,074
Provision for credit losses	5,580	(2,026)	6,457	(648)
Allowance for loan losses at period end	63,270	57,660	63,270	57,660
Allowance for unfunded lending-related commitments at period end	1,480	1,243	1,480	1,243
Allowance for credit losses at period end	64,750	58,903	64,750	58,903
Individually evaluated for impairment	3,979	3,190	3,979	3,190
Collectively evaluated for impairment	60,717	55,611	60,717	55,611
Loans at period end, Individually evaluated for impairment	33,749	34,588	33,749	34,588
Loans at period end, Collectively evaluated for impairment	7,115,504	6,433,615	7,115,504	6,433,615
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	102	54	102
Loans at period end, Loans acquired with deteriorated credit quality	126,991	106,881	126,991	106,881
Home equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	8,627	9,860	8,507	10,493
Other adjustments	(13)	0	(20)	0
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(690)	(612)	(778)	(969)
Recoveries	68	171	130	294
Provision for credit losses	(4,361)	132	(4,208)	(267)
Allowance for loan losses at period end	3,631	9,551	3,631	9,551
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	3,631	9,551	3,631	9,551
Individually evaluated for impairment	209	732	209	732
Collectively evaluated for impairment	3,422	8,819	3,422	8,819
Loans at period end, Individually evaluated for impairment	18,303	9,917	18,303	9,917
Loans at period end, Collectively evaluated for impairment	509,067	583,583	509,067	583,583
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	0	0
Loans at period end, Loans acquired with deteriorated credit quality	0	0	0	0
Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	7,630	6,278	7,194	6,688
Other adjustments	(8)	(2)	(15)	(5)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(287)	(180)	(290)	(751)
Recoveries	140	1,522	169	1,562
Provision for credit losses	671	(1,282)	1,088	(1,158)
Allowance for loan losses at period end	8,146	6,336	8,146	6,336
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	8,146	6,336	8,146	6,336
Individually evaluated for impairment	321	106	321	106
Collectively evaluated for impairment	7,762	6,199	7,762	6,199
Loans at period end, Individually evaluated for impairment	21,663	19,554	21,663	19,554
Loans at period end, Collectively evaluated for impairment	980,167	798,175	980,167	798,175
Loans held at fair value	106,081	67,181	106,081	67,181
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	31	63	31
Loans at period end, Loans acquired with deteriorated credit quality	10,267	10,560	10,267	10,560
Premium finance receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	8,219	7,333	7,715	6,846
Other adjustments	21	(14)	32	(26)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(5,009)	(3,254)	(7,219)	(7,975)
Recoveries	734	975	1,290	1,360
Provision for credit losses	4,975	2,694	7,122	7,529
Allowance for loan losses at period end	8,940	7,734	8,940	7,734
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	8,940	7,734	8,940	7,734
Individually evaluated for impairment	0	0	0	0
Collectively evaluated for impairment	8,940	7,734	8,940	7,734
Loans at period end, Individually evaluated for impairment	0	0	0	0
Loans at period end, Collectively evaluated for impairment	7,856,344	6,952,118	7,856,344	6,952,118
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	0	0
Loans at period end, Loans acquired with deteriorated credit quality	146,557	183,622	146,557	183,622
Consumer and other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan losses at beginning of period	1,248	915	1,261	840
Other adjustments	0	0	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(136)	(459)	(238)	(588)
Recoveries	60	49	116	96
Provision for credit losses	369	889	402	1,046
Allowance for loan losses at period end	1,541	1,394	1,541	1,394
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	1,541	1,394	1,541	1,394
Individually evaluated for impairment	109	120	109	120
Collectively evaluated for impairment	1,432	1,274	1,432	1,274
Loans at period end, Individually evaluated for impairment	271	603	271	603
Loans at period end, Collectively evaluated for impairment	106,276	119,399	106,276	119,399
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	0	0
Loans at period end, Loans acquired with deteriorated credit quality	$ 2,645	$ 1,704	$ 2,645	$ 1,704