Fair Values of Financial Instruments - Reconciliation and income statement classification using Level 3 inputs (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Investment income
Unrealized losses on investments	$ 377		$ 419
Collateralized debt obligations
Investment income
Unrealized losses on investments	20		27
Collateralized debt obligations \| Recurring \| Level 3
Reconciliation and income statement classification of gains and losses for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3)
Balance of recurring Level 3 assets at beginning of period	187	$ 199	199
Included in other comprehensive income (loss)	7	(13)
Principal maturities/settlements	(3)	(2)
Balance of recurring Level 3 assets at end of period	191	184	187
Investment income
Unrealized losses on investments	20		$ 27
Interest income	$ 4	$ 5

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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Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.

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