Fair Value Measurement - Narrative (Details)	9 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($) Security Trust	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Percentage of CDS contracts fair valued using minimum premium (de minimis)		51.00%
AGC And AGM
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of custodial trusts \| Trust	4
Share value, amount	$ 200,000,000
Maximum amount	$ 50,000,000
Recurring \| Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of fixed maturity securities valued using model processes \| Security	214
Fixed maturity securities	$ 1,316,000,000
Recurring \| Level 3 \| CDR \| Total \| Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	0.11%	0.19%
Recurring \| Level 3 \| CDR \| Total \| Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	1.98%	1.33%

Available-for-Sale Debt Securities and Short-Term Investments

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Represents the number of fixed maturity securities and short-term investments which have been valued using model processes set by the entity.

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Represents the number of custodial trusts with whom the entity entered into put agreements.

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Represents the percentage of CDS contracts which are fair valued using minimum premium. Minimum premium is the minimum rate that the company would currently charge to assume similar risks.

+ References

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Estimated rate of prepayment of principal on financial assets for initial fair value measurement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=121570589&loc=d3e107207-111719

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount that would be paid, determined under the conditions specified in the contract, if the holder of the share has the right to redeem the shares.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 480
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The maximum amount that the issuer could be required to pay to redeem the instrument by physical settlement, if applicable.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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