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Investment Securities - Summary of Cost, Gross Unrealized Holding Gains and Losses, Allowances for Credit Losses and Fair Value of Available-for-Sale Investments by Types, Maturities and Classes of Securities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Amortized Cost	$ 36,060	$ 33,556
Debt securities, Unrealized Gains	1	156
Debt securities, Unrealized Losses	(5)
Debt securities, Estimated Fair Value	$ 36,056	$ 33,712
Certificates of deposit | Within 1 Year
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Maturity	Within 1 year	Within 1 year
Debt securities, Amortized Cost	$ 2,853	$ 1,750
Debt securities, Unrealized Gains		4
Debt securities, Unrealized Losses	(1)
Debt securities, Estimated Fair Value	$ 2,852	$ 1,754
Certificates of deposit | 1 Year to 2 Years
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Maturity	1 year to 2 years
Debt securities, Amortized Cost	$ 3,137
Debt securities, Unrealized Losses	(2)
Debt securities, Estimated Fair Value	$ 3,135
U.S. Treasury securities | Within 1 Year
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Maturity	Within 1 year	Within 1 year
Debt securities, Amortized Cost	$ 10,066	$ 31,806
Debt securities, Unrealized Gains	1	152
Debt securities, Estimated Fair Value	$ 10,067	$ 31,958
U.S. Treasury securities | 1 Year to 2 Years
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Maturity	1 year to 2 years
Debt securities, Amortized Cost	$ 20,004
Debt securities, Unrealized Losses	(2)
Debt securities, Estimated Fair Value	$ 20,002

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Debt securities, available-for-sale, maturity.

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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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