Loans and Allowance for Loan Losses - Summary of Changes in Allowance for Loan Losses by Loan Category (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 111,080	$ 109,138	$ 113,031	$ 82,297	$ 82,297
Charge-offs	(2,663)	(1,264)	(4,645)	(3,607)
Recoveries	520	162	1,131	746
(Release of) provision	(3,300)	8,600	(3,880)	37,200
Ending balance	105,637	116,636	105,637	116,636	113,031
Allowance for loan losses ending balance:
Individually evaluated for impairment	7,368		7,368		8,054
Acquired with deteriorated credit quality	542		542		2,432
Collectively evaluated for impairment			97,727		102,545
Total allowance for loan losses by group	105,637	116,636	105,637	116,636	113,031
Loans ending balance:
Individually evaluated for impairment			71,565		75,609
Acquired with deteriorated credit quality	4,526		4,526		9,297
Collectively evaluated for impairment	9,544,984		9,544,984		9,645,619
Total loans	9,621,075		9,621,075		9,730,525
Commercial and Industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	25,406	30,531	26,617	20,919	20,919
Charge-offs	(550)	(27)	(550)	(27)
Recoveries	13	58	22	380
(Release of) provision	(2,273)	2,667	(3,493)	11,957
Ending balance	22,596	33,229	22,596	33,229	26,617
Allowance for loan losses ending balance:
Individually evaluated for impairment	4,544		4,544		4,555
Acquired with deteriorated credit quality	0		0		1,283
Collectively evaluated for impairment			18,052		20,779
Total allowance for loan losses by group	22,596	33,229	22,596	33,229	26,617
Loans ending balance:
Individually evaluated for impairment			20,266		17,343
Acquired with deteriorated credit quality	1,397		1,397		3,432
Collectively evaluated for impairment	1,719,016		1,719,016		1,974,241
Total loans	1,740,679		1,740,679		1,995,016
Commercial Real Estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	55,138	49,227	54,569	34,730	34,730
Charge-offs	0	(24)	(234)	(24)
Recoveries	4	5	4	6
(Release of) provision	(2,383)	5,020	(1,580)	19,516
Ending balance	52,759	54,228	52,759	54,228	54,569
Allowance for loan losses ending balance:
Individually evaluated for impairment	0		0		210
Acquired with deteriorated credit quality	249		249		822
Collectively evaluated for impairment			52,510		53,537
Total allowance for loan losses by group	52,759	54,228	52,759	54,228	54,569
Loans ending balance:
Individually evaluated for impairment			4,051		4,435
Acquired with deteriorated credit quality	249		249		2,749
Collectively evaluated for impairment	3,771,471		3,771,471		3,566,446
Total loans	3,775,771		3,775,771		3,573,630
Commercial Construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3,350	4,712	4,553	3,424	3,424
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
(Release of) provision	96	104	(1,107)	1,392
Ending balance	3,446	4,816	3,446	4,816	4,553
Allowance for loan losses ending balance:
Individually evaluated for impairment	0		0		0
Acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment			3,446		4,553
Total allowance for loan losses by group	3,446	4,816	3,446	4,816	4,553
Loans ending balance:
Individually evaluated for impairment			0		0
Acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment	237,927		237,927		305,708
Total loans	237,927		237,927		305,708
Business Banking
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	13,504	10,181	13,152	8,260	8,260
Charge-offs	(1,838)	(1,198)	(3,222)	(2,535)
Recoveries	291	27	656	154
(Release of) provision	748	795	2,119	3,926
Ending balance	12,705	9,805	12,705	9,805	13,152
Allowance for loan losses ending balance:
Individually evaluated for impairment	937		937		1,435
Acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment			11,768		11,717
Total allowance for loan losses by group	12,705	9,805	12,705	9,805	13,152
Loans ending balance:
Individually evaluated for impairment			18,179		21,901
Acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment	1,321,673		1,321,673		1,317,263
Total loans	1,339,852		1,339,852		1,339,164
Residential Real Estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	6,235	6,228	6,435	6,380	6,380
Charge-offs	0	0	0	0
Recoveries	17	13	27	73
(Release of) provision	226	328	16	116
Ending balance	6,478	6,569	6,478	6,569	6,435
Allowance for loan losses ending balance:
Individually evaluated for impairment	1,624		1,624		1,565
Acquired with deteriorated credit quality	293		293		327
Collectively evaluated for impairment			4,561		4,543
Total allowance for loan losses by group	6,478	6,569	6,478	6,569	6,435
Loans ending balance:
Individually evaluated for impairment			25,091		27,056
Acquired with deteriorated credit quality	2,880		2,880		3,116
Collectively evaluated for impairment	1,429,527		1,429,527		1,340,785
Total loans	1,457,498		1,457,498		1,370,957
Consumer Home Equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3,576	3,913	3,744	4,027	4,027
Charge-offs	0	0	0	(473)
Recoveries	3	8	74	22
(Release of) provision	9	(46)	(230)	299
Ending balance	3,588	3,875	3,588	3,875	3,744
Allowance for loan losses ending balance:
Individually evaluated for impairment	263		263		289
Acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment			3,325		3,455
Total allowance for loan losses by group	3,588	3,875	3,588	3,875	3,744
Loans ending balance:
Individually evaluated for impairment			3,954		4,845
Acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment	830,984		830,984		863,425
Total loans	834,938		834,938		868,270
Other Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3,498	4,019	3,467	4,173	4,173
Charge-offs	(275)	(15)	(639)	(548)
Recoveries	192	51	348	111
(Release of) provision	211	(293)	450	26
Ending balance	3,626	3,762	3,626	3,762	3,467
Allowance for loan losses ending balance:
Individually evaluated for impairment	0		0		0
Acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment			3,626		3,467
Total allowance for loan losses by group	3,626	3,762	3,626	3,762	3,467
Loans ending balance:
Individually evaluated for impairment			24		29
Acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment	234,386		234,386		277,751
Total loans	234,410		234,410		277,780
Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	373	327	494	384	384
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
(Release of) provision	66	25	(55)	(32)
Ending balance	439	352	439	352	494
Allowance for loan losses ending balance:
Individually evaluated for impairment	0		0		0
Acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment			439		494
Total allowance for loan losses by group	439	$ 352	439	$ 352	494
Loans ending balance:
Individually evaluated for impairment			0		0
Acquired with deteriorated credit quality	0		0		0
Collectively evaluated for impairment	0		0		0
Total loans	$ 0		$ 0		$ 0