Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Summary of Impaired Loans by Loan Class) (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	$ 83,349	$ 30,864	$ 36,084
Impaired Financing Receivable, Related Allowance	14,365	7,218	8,023
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	49,173	47,730	69,004
Total impaired loans	132,522	78,594	105,088
Impaired Financing Receivable, Unpaid Principal Balance	145,561	91,091	127,566
Impaired Financing Receivable, Average Recorded Investment	134,975	84,905	115,261
Impaired Financing Receivable, Interest Income Recognized	5,768	3,424	6,298
Commercial
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance		0
Impaired Financing Receivable, Related Allowance		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment		0
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized		0
Impaired Financing Receivable, with No Related Allowance, Recorded Investment		0
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance		0
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment		0
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized		0
Commercial | Commercial, industrial and other
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	36,564	7,312	6,233
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	36,699	8,458	7,323
Impaired Financing Receivable, Related Allowance	8,242	4,191	3,951
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	30,259	8,390	7,220
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	1,404	407	452
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	5,758	5,995	8,460
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	7,022	7,260	12,259
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	9,325	6,662	10,170
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	465	294	683
Commercial | Franchise
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	16,316		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	18,504		0
Impaired Financing Receivable, Related Allowance	1,638		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	18,387		0
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	771		0
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	5,157		16,256
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	5,158		16,256
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	5,376		17,089
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	302		780
Commercial | Asset-based lending
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	646	588	948
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	646	589	949
Impaired Financing Receivable, Related Allowance	283	161	355
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	724	588	1,302
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	39	21	72
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	264	553	602
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	1,088	553	602
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	1,623	728	688
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	89	31	40
Commercial | Leases
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,777	2,440	2,331
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	1,777	2,444	2,337
Impaired Financing Receivable, Related Allowance	1	215	158
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	1,836	2,539	2,463
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	69	91	117
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	899	817	782
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	930	817	782
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	974	862	845
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	43	38	49
Commercial real estate | Construction
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,554	1,607	3,097
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	1,554	2,408	3,897
Impaired Financing Receivable, Related Allowance	390	94	403
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	1,554	2,319	3,690
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	56	93	197
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,117	1,504	1,367
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	1,117	1,504	1,678
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	1,252	1,524	1,555
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	40	49	84
Commercial real estate | Land
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,375	0	0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	1,375	0	0
Impaired Financing Receivable, Related Allowance	1	0	0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	1,508	0	0
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	53	0	0
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	2,325	3,968	3,961
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	2,431	4,217	4,192
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	2,366	4,110	4,129
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	98	136	182
Commercial real estate | Office
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	579	2,225	471
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	647	2,291	471
Impaired Financing Receivable, Related Allowance	26	570	5
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	591	2,280	481
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	19	94	24
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,532	3,400	2,438
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	2,077	3,585	6,140
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	1,541	3,565	3,484
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	86	147	330
Commercial real estate | Industrial
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	45	408	408
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	154	408	408
Impaired Financing Receivable, Related Allowance	1	75	40
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	56	415	414
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	6	19	25
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	178	1,440	403
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	195	2,729	2,010
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	188	2,885	1,849
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	9	183	174
Commercial real estate | Retail
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	15,325	5,932	15,599
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	15,567	6,072	15,657
Impaired Financing Receivable, Related Allowance	2,413	158	1,336
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	15,376	5,998	15,736
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	535	191	624
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	777	1,978	2,393
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	946	1,988	3,538
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	874	2,008	2,486
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	42	103	221
Commercial real estate | Multi-family
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,197	1,239	0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	1,197	1,239	0
Impaired Financing Receivable, Related Allowance	8	8	0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	1,209	1,250	0
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	32	33	0
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	318	569	1,231
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	412	653	2,078
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	329	571	1,246
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	9	23	76
Commercial real estate | Mixed use and other
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,590	1,537	1,567
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	1,801	1,695	1,586
Impaired Financing Receivable, Related Allowance	309	263	379
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	1,754	1,580	1,599
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	76	60	77
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	3,763	5,546	5,275
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	4,362	6,267	6,731
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	3,950	5,745	5,559
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	194	241	351
Home equity
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	2,287	1,511	1,606
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	2,651	1,721	1,869
Impaired Financing Receivable, Related Allowance	611	691	784
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	2,303	1,528	1,626
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	78	53	81
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	8,997	6,218	7,648
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	12,131	9,523	11,648
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	9,015	7,231	9,114
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	462	339	603
Residential real estate
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	3,977	5,842	3,798
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	4,291	6,154	3,910
Impaired Financing Receivable, Related Allowance	325	758	586
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	3,998	5,842	3,790
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	128	177	146
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	17,804	15,421	17,455
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	20,291	17,859	20,327
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	18,193	15,726	17,926
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	643	575	860
Consumer and other
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	117	223	26
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	130	224	28
Impaired Financing Receivable, Related Allowance	117	34	26
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	119	225	27
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	5	10	2
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	284	321	733
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	408	433	890
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	295	334	773
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	$ 15	$ 16	$ 48