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Fair Value Measurement - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2022 USD ($) Trust Security	Dec. 31, 2021
Assured IM
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Obligation to purchase debt	0.05
AGC And AGM
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of custodial trusts | Trust	4
Share value, amount	$ 200
Maximum amount	$ 50
Recurring | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Number of fixed maturity securities valued using model processes | Security	190
Fixed maturity securities	$ 1,100
Recurring | Level 3 | CDR | Total | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	0.45%	0.11%
Recurring | Level 3 | CDR | Total | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	2.93%	1.78%

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

+ 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)
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- Definition

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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

The maximum amount that the issuer could be required to pay to redeem the instrument by physical settlement, if applicable.

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