Investment securities - Schedule of Present Cost or Amortized Cost and Fair Values of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Investment securities, held-to-maturity:
Amortized cost	$ 695	$ 694
Gross unrealized gains	42	43
Gross unrealized losses	(10)	0
Fair value	727	737
Investment securities, available-for-sale:
Amortized cost	167,935	53,953
Gross unrealized gains	37	51
Gross unrealized losses	(647)	(41)
Fair value	167,325	53,963
Total investment securities
Amortized cost	168,630	54,647	$ 57,045
Gross unrealized gains	79	94	68
Gross unrealized losses	(657)	(41)	0
Fair value	$ 168,052	54,700	57,113
U.S. government and government agencies and authorities \| Investment securities, held-to-maturity:
Investment securities, held-to-maturity:
Amortized cost		694	663
Gross unrealized gains		43	22
Gross unrealized losses		0	0
Fair value		737	685
U.S. government and government agencies and authorities \| Investment securities, available-for-sale:
Investment securities, available-for-sale:
Amortized cost		53,953	56,382
Gross unrealized gains		51	46
Gross unrealized losses		(41)	0
Fair value		$ 53,963	$ 56,428

Available for sale securities and held to maturity securities accumulated gross unrealized gain before tax.

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Available for sale securities and held to maturity securities accumulated gross unrealized loss before tax.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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Sum of the carrying amounts as of the balance sheet date of all investments.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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