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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	$ 139,503	$ 125,819	$ 137,905	$ 122,291
Other adjustments	(44)	(30)	(84)	(86)
Reclassification from allowance for unfunded lending-related commitments	0	106	26	(32)
Charge-offs	(6,870)	(6,728)	(16,148)	(10,145)
Recoveries	5,770	1,472	8,314	3,295
Provision for credit losses	5,043	8,952	13,389	14,268
Allowance for loan losses at period end	143,402	129,591	143,402	129,591
Allowance for unfunded lending-related commitments at period end	1,243	1,705	1,243	1,705
Allowance for credit losses at period end	144,645	131,296	144,645	131,296
Individually evaluated for impairment	8,554	6,152	8,554	6,152
Collectively evaluated for impairment	135,471	124,314	135,471	124,314
Loans at period end, Individually evaluated for impairment	120,610	80,072	120,610	80,072
Loans at period end, Collectively evaluated for impairment	22,111,452	20,245,483	22,111,452	20,245,483
Loans held at fair value	67,181	30,173	67,181	30,173
Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	620	830	620	830
Loans at period end, Loans acquired with deteriorated credit quality	311,317	387,604	311,317	387,604
Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	57,636	46,582	57,811	44,493
Other adjustments	(1)	(2)	(2)	(21)
Reclassification from allowance for unfunded lending-related commitments	0	92	0	0
Charge-offs	(2,210)	(913)	(4,897)	(1,554)
Recoveries	666	561	928	834
Provision for credit losses	4,636	6,038	6,887	8,606
Allowance for loan losses at period end	60,727	52,358	60,727	52,358
Allowance for unfunded lending-related commitments at period end	0	500	0	500
Allowance for credit losses at period end	60,727	52,858	60,727	52,858
Individually evaluated for impairment	4,406	2,528	4,406	2,528
Collectively evaluated for impairment	55,834	49,692	55,834	49,692
Loans at period end, Individually evaluated for impairment	55,948	14,469	55,948	14,469
Loans at period end, Collectively evaluated for impairment	7,224,562	6,381,610	7,224,562	6,381,610
Loans held at fair value	0	0	0	0
Commercial | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	487	638	487	638
Loans at period end, Loans acquired with deteriorated credit quality	8,550	10,210	8,550	10,210
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	57,481	52,633	55,227	51,422
Other adjustments	(27)	(47)	(51)	(83)
Reclassification from allowance for unfunded lending-related commitments	0	14	26	(32)
Charge-offs	(155)	(1,985)	(968)	(2,246)
Recoveries	2,387	276	4,074	830
Provision for credit losses	(2,026)	1,448	(648)	2,448
Allowance for loan losses at period end	57,660	52,339	57,660	52,339
Allowance for unfunded lending-related commitments at period end	1,243	1,205	1,243	1,205
Allowance for credit losses at period end	58,903	53,544	58,903	53,544
Individually evaluated for impairment	3,190	1,473	3,190	1,473
Collectively evaluated for impairment	55,611	51,952	55,611	51,952
Loans at period end, Individually evaluated for impairment	34,588	34,690	34,588	34,690
Loans at period end, Collectively evaluated for impairment	6,433,615	6,221,567	6,433,615	6,221,567
Loans held at fair value	0	0	0	0
Commercial real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	102	119	102	119
Loans at period end, Loans acquired with deteriorated credit quality	106,881	146,237	106,881	146,237
Home equity
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	9,860	12,203	10,493	11,774
Other adjustments	0	0	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(612)	(1,631)	(969)	(2,256)
Recoveries	171	144	294	209
Provision for credit losses	132	418	(267)	1,407
Allowance for loan losses at period end	9,551	11,134	9,551	11,134
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	9,551	11,134	9,551	11,134
Individually evaluated for impairment	732	1,296	732	1,296
Collectively evaluated for impairment	8,819	9,838	8,819	9,838
Loans at period end, Individually evaluated for impairment	9,917	9,633	9,917	9,633
Loans at period end, Collectively evaluated for impairment	583,583	679,850	583,583	679,850
Loans held at fair value	0	0	0	0
Home equity | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [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 Losses [Roll Forward]
Allowance for loan losses at beginning of period	6,278	5,530	6,688	5,714
Other adjustments	(2)	(3)	(5)	(7)
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(180)	(146)	(751)	(475)
Recoveries	1,522	54	1,562	232
Provision for credit losses	(1,282)	708	(1,158)	679
Allowance for loan losses at period end	6,336	6,143	6,336	6,143
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	6,336	6,143	6,336	6,143
Individually evaluated for impairment	106	764	106	764
Collectively evaluated for impairment	6,199	5,306	6,199	5,306
Loans at period end, Individually evaluated for impairment	19,554	20,859	19,554	20,859
Loans at period end, Collectively evaluated for impairment	798,175	708,042	798,175	708,042
Loans held at fair value	67,181	30,173	67,181	30,173
Residential real estate | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	31	73	31	73
Loans at period end, Loans acquired with deteriorated credit quality	10,560	3,736	10,560	3,736
Premium finance receivables
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	7,333	7,559	6,846	7,625
Other adjustments	(14)	22	(26)	25
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(3,254)	(1,878)	(7,975)	(3,305)
Recoveries	975	404	1,360	1,016
Provision for credit losses	2,694	245	7,529	991
Allowance for loan losses at period end	7,734	6,352	7,734	6,352
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	7,734	6,352	7,734	6,352
Individually evaluated for impairment	0	0	0	0
Collectively evaluated for impairment	7,734	6,352	7,734	6,352
Loans at period end, Individually evaluated for impairment	0	0	0	0
Loans at period end, Collectively evaluated for impairment	6,952,118	6,141,095	6,952,118	6,141,095
Loans held at fair value	0	0	0	0
Premium finance receivables | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	0	0	0	0
Loans at period end, Loans acquired with deteriorated credit quality	183,622	226,334	183,622	226,334
Consumer and other
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	915	1,312	840	1,263
Other adjustments	0	0	0	0
Reclassification from allowance for unfunded lending-related commitments	0	0	0	0
Charge-offs	(459)	(175)	(588)	(309)
Recoveries	49	33	96	174
Provision for credit losses	889	95	1,046	137
Allowance for loan losses at period end	1,394	1,265	1,394	1,265
Allowance for unfunded lending-related commitments at period end	0	0	0	0
Allowance for credit losses at period end	1,394	1,265	1,394	1,265
Individually evaluated for impairment	120	91	120	91
Collectively evaluated for impairment	1,274	1,174	1,274	1,174
Loans at period end, Individually evaluated for impairment	603	421	603	421
Loans at period end, Collectively evaluated for impairment	119,399	113,319	119,399	113,319
Loans held at fair value	0	0	0	0
Consumer and other | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses at period end	0	0	0	0
Loans at period end, Loans acquired with deteriorated credit quality	$ 1,704	$ 1,087	$ 1,704	$ 1,087