Loans and Allowance for Loan Losses - Summary of Changes in Allowance for Loan Losses by Loan Category (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of period	$ 113,031	$ 82,297	$ 82,297
Allowance for loan losses, Charge-offs	(1,982)	(2,343)
Allowance for loan losses, Recoveries	611	584
Allowance for loan losses, Provision (benefit)	(580)	28,600
Balance at end of period	111,080	109,138	113,031
Ending balance: individually evaluated for impairment	8,137	6,256
Ending balance: acquired with deteriorated credit quality	2,432	1,192
Ending balance: collectively evaluated for impairment	100,511	101,690
Loans ending balance:
Individually evaluated for impairment	74,390	86,431
Acquired with deteriorated credit quality	7,617	13,143
Collectively evaluated for impairment	9,834,468	8,987,529
Total loans	9,916,475	9,087,103	9,730,525
Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of period	26,617	20,919	20,919
Allowance for loan losses, Charge-offs	0	0
Allowance for loan losses, Recoveries	9	322
Allowance for loan losses, Provision (benefit)	(1,220)	9,290
Balance at end of period	25,406	30,531	26,617
Ending balance: individually evaluated for impairment	4,761	4,037
Ending balance: acquired with deteriorated credit quality	1,283	0
Ending balance: collectively evaluated for impairment	19,362	26,494
Loans ending balance:
Individually evaluated for impairment	17,907	32,423
Acquired with deteriorated credit quality	2,944	3,939
Collectively evaluated for impairment	1,965,515	1,734,760
Total loans	1,986,366	1,771,122	1,995,016
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of period	54,569	34,730	34,730
Allowance for loan losses, Charge-offs	(234)	0
Allowance for loan losses, Recoveries	0	1
Allowance for loan losses, Provision (benefit)	803	14,496
Balance at end of period	55,138	49,227	54,569
Ending balance: individually evaluated for impairment	210	270
Ending balance: acquired with deteriorated credit quality	822	936
Ending balance: collectively evaluated for impairment	54,106	48,021
Loans ending balance:
Individually evaluated for impairment	4,536	8,054
Acquired with deteriorated credit quality	1,554	5,780
Collectively evaluated for impairment	3,670,851	3,509,887
Total loans	3,676,941	3,523,721	3,573,630
Commercial construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of period	4,553	3,424	3,424
Allowance for loan losses, Charge-offs	0	0
Allowance for loan losses, Recoveries	0	0
Allowance for loan losses, Provision (benefit)	(1,203)	1,288
Balance at end of period	3,350	4,712	4,553
Ending balance: individually evaluated for impairment	0	0
Ending balance: acquired with deteriorated credit quality	0	0
Ending balance: collectively evaluated for impairment	3,350	4,712
Loans ending balance:
Individually evaluated for impairment	0	0
Acquired with deteriorated credit quality	0	0
Collectively evaluated for impairment	249,416	293,135
Total loans	249,416	293,135	305,708
Business banking
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of period	13,152	8,260	8,260
Allowance for loan losses, Charge-offs	(1,384)	(1,337)
Allowance for loan losses, Recoveries	365	127
Allowance for loan losses, Provision (benefit)	1,371	3,131
Balance at end of period	13,504	10,181	13,152
Ending balance: individually evaluated for impairment	1,387	453
Ending balance: acquired with deteriorated credit quality	0	0
Ending balance: collectively evaluated for impairment	12,117	9,728
Loans ending balance:
Individually evaluated for impairment	21,001	10,258
Acquired with deteriorated credit quality	0	0
Collectively evaluated for impairment	1,492,050	769,658
Total loans	1,513,051	779,916	1,339,164
Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of period	6,435	6,380	6,380
Allowance for loan losses, Charge-offs	0	0
Allowance for loan losses, Recoveries	10	60
Allowance for loan losses, Provision (benefit)	(210)	(212)
Balance at end of period	6,235	6,228	6,435
Ending balance: individually evaluated for impairment	1,516	1,275
Ending balance: acquired with deteriorated credit quality	327	256
Ending balance: collectively evaluated for impairment	4,392	4,697
Loans ending balance:
Individually evaluated for impairment	26,459	29,393
Acquired with deteriorated credit quality	3,119	3,424
Collectively evaluated for impairment	1,376,932	1,387,186
Total loans	1,406,510	1,420,003	1,370,957
Consumer home equity
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of period	3,744	4,027	4,027
Allowance for loan losses, Charge-offs	0	(473)
Allowance for loan losses, Recoveries	71	14
Allowance for loan losses, Provision (benefit)	(239)	345
Balance at end of period	3,576	3,913	3,744
Ending balance: individually evaluated for impairment	263	221
Ending balance: acquired with deteriorated credit quality	0	0
Ending balance: collectively evaluated for impairment	3,313	3,692
Loans ending balance:
Individually evaluated for impairment	4,461	6,280
Acquired with deteriorated credit quality	0	0
Collectively evaluated for impairment	828,005	923,274
Total loans	832,466	929,554	868,270
Other consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of period	3,467	4,173	4,173
Allowance for loan losses, Charge-offs	(364)	(533)
Allowance for loan losses, Recoveries	156	60
Allowance for loan losses, Provision (benefit)	239	319
Balance at end of period	3,498	4,019	3,467
Ending balance: individually evaluated for impairment	0	0
Ending balance: acquired with deteriorated credit quality	0	0
Ending balance: collectively evaluated for impairment	3,498	4,019
Loans ending balance:
Individually evaluated for impairment	26	23
Acquired with deteriorated credit quality	0	0
Collectively evaluated for impairment	251,699	369,629
Total loans	251,725	369,652	277,780
Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at the beginning of period	494	384	384
Allowance for loan losses, Charge-offs	0	0
Allowance for loan losses, Recoveries	0	0
Allowance for loan losses, Provision (benefit)	(121)	(57)
Balance at end of period	373	327	$ 494
Ending balance: individually evaluated for impairment	0	0
Ending balance: acquired with deteriorated credit quality	0	0
Ending balance: collectively evaluated for impairment	373	327
Loans ending balance:
Individually evaluated for impairment	0	0
Acquired with deteriorated credit quality	0	0
Collectively evaluated for impairment	0	0
Total loans	$ 0	$ 0