Allowance for Credit Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) SecurityLoan	Dec. 31, 2020 USD ($) SecurityLoan	Dec. 31, 2019 USD ($)
Financing Receivable, Past Due [Line Items]
Total	$ 6,970,087	$ 7,225,860
Non - performing	111,312	146,774	$ 54,057
Current
Financing Receivable, Past Due [Line Items]
Total	6,854,864	7,069,730
Total Past Due Loans
Financing Receivable, Past Due [Line Items]
Total	115,223	156,130
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total	2,022	2,888
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total	1,889	5,965
Past Due 90+ Days Still Accruing
Financing Receivable, Past Due [Line Items]
Total		503
Commercial
Financing Receivable, Past Due [Line Items]
Total	5,423,964	$ 5,673,707
Non - performing	$ 1,700
Commercial | CARES Act, Paycheck Protection Program Loans
Financing Receivable, Past Due [Line Items]
Number of modified loans | SecurityLoan	42	52
Modified loans	$ 58,900	$ 195,600
Commercial | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total	2,702,780	2,791,947
Non - performing	54,797	101,070	25,356
Commercial | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total	1,478,198	1,559,552
Non - performing	33,909	16,985	10,911
Commercial | Commercial construction
Financing Receivable, Past Due [Line Items]
Total	485,924	466,077
Non - performing	4,053	384	737
Commercial | Business banking
Financing Receivable, Past Due [Line Items]
Total	1,100,125	1,160,067
Non - performing	10,221	17,122	9,863
Commercial | Current | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total	2,647,983	2,690,877
Commercial | Current | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total	1,444,289	1,542,567
Commercial | Current | Commercial construction
Financing Receivable, Past Due [Line Items]
Total	481,871	462,094
Commercial | Current | Business banking
Financing Receivable, Past Due [Line Items]
Total	1,088,013	1,140,581
Commercial | Total Past Due Loans | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total	54,797	101,070
Commercial | Total Past Due Loans | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total	33,909	16,985
Commercial | Total Past Due Loans | Commercial construction
Financing Receivable, Past Due [Line Items]
Total	4,053	3,983
Commercial | Total Past Due Loans | Business banking
Financing Receivable, Past Due [Line Items]
Total	12,112	19,486
Commercial | 30-59 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total	0	0
Commercial | 30-59 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total	0	0
Commercial | 30-59 Days Past Due | Commercial construction
Financing Receivable, Past Due [Line Items]
Total	0	19
Commercial | 30-59 Days Past Due | Business banking
Financing Receivable, Past Due [Line Items]
Total	970	1,614
Commercial | 60-89 Days Past Due | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total	0	0
Commercial | 60-89 Days Past Due | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total	0	0
Commercial | 60-89 Days Past Due | Commercial construction
Financing Receivable, Past Due [Line Items]
Total	0	3,580
Commercial | 60-89 Days Past Due | Business banking
Financing Receivable, Past Due [Line Items]
Total	921	379
Commercial | Past Due 90+ Days Still Accruing | Commercial real estate
Financing Receivable, Past Due [Line Items]
Total		0
Commercial | Past Due 90+ Days Still Accruing | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Total		0
Commercial | Past Due 90+ Days Still Accruing | Commercial construction
Financing Receivable, Past Due [Line Items]
Total		0
Commercial | Past Due 90+ Days Still Accruing | Business banking
Financing Receivable, Past Due [Line Items]
Total		371
Consumer
Financing Receivable, Past Due [Line Items]
Total	1,546,123	1,552,153
Consumer | Business banking
Financing Receivable, Past Due [Line Items]
Total		303,900
Consumer | Consumer real estate
Financing Receivable, Past Due [Line Items]
Total	1,105,478	1,167,332
Non - performing	8,143	11,117	6,063
Consumer | Other consumer
Financing Receivable, Past Due [Line Items]
Total	97,582	80,885
Non - performing	189	96	$ 1,127
Consumer | Current | Consumer real estate
Financing Receivable, Past Due [Line Items]
Total	1,095,557	1,153,028
Consumer | Current | Other consumer
Financing Receivable, Past Due [Line Items]
Total	97,151	80,583
Consumer | Total Past Due Loans | Consumer real estate
Financing Receivable, Past Due [Line Items]
Total	9,921	14,304
Consumer | Total Past Due Loans | Other consumer
Financing Receivable, Past Due [Line Items]
Total	431	302
Consumer | 30-59 Days Past Due | Consumer real estate
Financing Receivable, Past Due [Line Items]
Total	833	1,087
Consumer | 30-59 Days Past Due | Other consumer
Financing Receivable, Past Due [Line Items]
Total	219	168
Consumer | 60-89 Days Past Due | Consumer real estate
Financing Receivable, Past Due [Line Items]
Total	945	1,968
Consumer | 60-89 Days Past Due | Other consumer
Financing Receivable, Past Due [Line Items]
Total	$ 23	37
Consumer | Past Due 90+ Days Still Accruing | Consumer real estate
Financing Receivable, Past Due [Line Items]
Total		132
Consumer | Past Due 90+ Days Still Accruing | Other consumer
Financing Receivable, Past Due [Line Items]
Total		$ 0