Derivative Instruments and Hedging Activities- Narrative (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) instrument	Dec. 31, 2014 USD ($)	Dec. 31, 2020 USD ($)
Derivative [Line Items]
Loans receivable with fixed rates of interest maturity period		10 years
Loans receivable with fixed rates		$ 29,100,000
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount	$ 14,828,000		$ 15,465,000
Derivative liabilities	309,000		533,000
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount	902,047,000		833,671,000
Derivative liabilities	352,000		1,295,000
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount	$ 80,000		179,000
Loans receivable with fixed rates of interest maturity period	15 years
Derivative fixed interest rate	7.43%
Loans receivable fixed interest rate (percentage)	7.43%
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member] | Other Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	$ 2,000		8,000
Credit Risk Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount	$ 762,420,000		643,556,000
Derivative number of instruments held | instrument	123
Credit Risk Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	$ 350,000		535,000
Underlying derivative at fair value	350,000
Cash Flow Hedging [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount		$ 20,000,000
Derivative fixed interest rate		2.10%
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months	228,000
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount	14,828,000		15,465,000
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Other Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	$ 309,000		$ 533,000
Minimum [Member] | Credit Risk Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative remaining maturity	6 months
Maximum [Member] | Credit Risk Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative remaining maturity	10 years
One-Month LIBOR [Member] | Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Basis points	2.24%
Customer [Member] | Credit Risk Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Underlying derivative at fair value	$ 18,200,000