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	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	$ 37,572	$ 39,968	$ 36,084
Impaired Financing Receivable, Related Allowance	6,863	8,165	8,023
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	65,559	47,554	69,004
Total impaired loans	103,131	87,522	105,088
Impaired Financing Receivable, Unpaid Principal Balance	116,304	96,690	127,566
Impaired Financing Receivable, Average Recorded Investment	103,951	88,405	115,261
Impaired Financing Receivable, Interest Income Recognized	1,446	1,215	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	5,521	3,804	6,233
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	5,587	3,830	7,323
Impaired Financing Receivable, Related Allowance	1,738	1,568	3,951
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	5,607	3,856	7,220
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	83	50	452
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	5,480	10,270	8,460
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	6,777	11,307	12,259
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	5,650	10,668	10,170
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	109	185	683
Commercial | Franchise
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0		0
Impaired Financing Receivable, Related Allowance	0		0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	0		0
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	0		0
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	18,657		16,256
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	18,661		16,256
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	18,675		17,089
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	239		780
Commercial | Asset-based lending
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,107	1,378	948
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	1,107	1,380	949
Impaired Financing Receivable, Related Allowance	475	378	355
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	1,166	1,279	1,302
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	20	12	72
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	86		602
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	231		602
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	182		688
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	3		40
Commercial | Leases
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	2,213	2,616	2,331
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	2,221	2,619	2,337
Impaired Financing Receivable, Related Allowance	131	304	158
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	2,247	2,689	2,463
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	27	36	117
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	746	846	782
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	746	846	782
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	754	852	845
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	11	13	49
Commercial real estate | Construction
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	3,097	5,262	3,097
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	3,897	5,262	3,897
Impaired Financing Receivable, Related Allowance	599	74	403
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	3,097	5,276	3,690
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	50	53	197
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,363	3,912	1,367
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	1,364	3,912	1,678
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	1,364	3,973	1,555
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	15	46	84
Commercial real estate | Land
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,500	3,033	0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	1,500	3,033	0
Impaired Financing Receivable, Related Allowance	3	13	0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	1,567	3,033	0
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	17	28	0
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	2,329	1,240	3,961
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	2,434	1,631	4,192
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	2,339	1,321	4,129
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	31	15	182
Commercial real estate | Office
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,479	1,512	471
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	2,078	1,522	471
Impaired Financing Receivable, Related Allowance	73	310	5
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	1,483	1,513	481
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	24	18	24
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	59	2,487	2,438
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	754	3,803	6,140
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	61	2,432	3,484
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	11	56	330
Commercial real estate | Industrial
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	63	4,831	408
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	172	5,554	408
Impaired Financing Receivable, Related Allowance	1	1,703	40
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	63	4,854	414
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	2	71	25
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,427	2,172	403
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	1,485	2,487	2,010
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	1,430	2,152	1,849
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	20	57	174
Commercial real estate | Retail
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	15,347	1,733	15,599
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	15,415	1,843	15,657
Impaired Financing Receivable, Related Allowance	2,512	156	1,336
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	15,315	1,739	15,736
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	166	23	624
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	2,695	0	2,393
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	2,992	0	3,538
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	2,710	0	2,486
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	58	0	221
Commercial real estate | Multi-family
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,234	1,256	0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	1,277	1,256	0
Impaired Financing Receivable, Related Allowance	21	20	0
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	1,254	1,256	0
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	12	11	0
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	0	668	1,231
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	84	752	2,078
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	0	652	1,246
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	1	11	76
Commercial real estate | Mixed use and other
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	2,036	5,472	1,567
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	2,281	5,561	1,586
Impaired Financing Receivable, Related Allowance	388	902	379
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	2,054	5,486	1,599
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	30	67	77
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	5,284	6,153	5,275
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	5,981	6,961	6,731
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	5,340	6,234	5,559
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	80	91	351
Home equity
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,697	3,863	1,606
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	1,889	3,891	1,869
Impaired Financing Receivable, Related Allowance	749	1,979	784
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	1,699	3,866	1,626
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	19	35	81
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	8,405	8,015	7,648
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	12,535	10,420	11,648
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	8,255	8,176	9,114
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	151	123	603
Residential real estate
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	2,253	5,116	3,798
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	2,956	5,652	3,910
Impaired Financing Receivable, Related Allowance	148	666	586
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	2,258	5,166	3,790
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	33	57	146
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	18,305	11,478	17,455
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	20,983	12,673	20,327
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	18,630	11,522	17,926
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	222	151	860
Consumer and other
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, with Related Allowance, Recorded Investment	25	92	26
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	27	94	28
Impaired Financing Receivable, Related Allowance	25	92	26
Impaired Financing Receivable, with Related Allowance, Average Recorded Investment	25	95	27
Impaired Financing Receivable, with Related Allowance, Interest Income Recognized	0	1	2
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	723	313	733
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	870	401	890
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	726	315	773
Impaired Financing Receivable, with No Related Allowance, Interest Income Recognized	$ 12	$ 5	$ 48