Investments - Additional Information (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2022 USD ($) Security	Jun. 30, 2022 USD ($) Security manager	Jun. 30, 2021 USD ($)	Dec. 31, 2021 USD ($) Security
Schedule of Investments [Line Items]
Number of outside managers managing investment portfolio | manager		3
Accrued investment income	$ 74	$ 74		$ 69
Number of securities with unrealized losses greater than 10% of book value for 12 months or more | Security	435	435		23
Total unrealized losses for securities having losses greater than 10% of book value for 12 months or more	$ 211	$ 211		$ 6
Assets held-in-trust	217	217		243
PCD Accounting Model to PCI Securities
Schedule of Investments [Line Items]
Allowance for credit loss	4	9	$ 3
AGL Subsidiaries
Schedule of Investments [Line Items]
Assets held-in-trust	1,149	1,149		1,231
Future Equity Investments
Schedule of Investments [Line Items]
Remaining minimum amount committed	96	96
Future Equity Investments | AGL Subsidiaries | AssuredIM managed fixed-maturity securities
Schedule of Investments [Line Items]
Investments, authorized amount to invest	750	750
Investments, including distributed gains, authorized amount to invest	810	810
Invested Capital | AGL Subsidiaries | AssuredIM managed fixed-maturity securities
Schedule of Investments [Line Items]
Long-term purchase commitment, amount		516
Unfunded Commitment | AGL Subsidiaries | AssuredIM managed fixed-maturity securities
Schedule of Investments [Line Items]
Long-term purchase commitment, amount		241
Equity in Earnings (Losses) of Investees | AGL Subsidiaries | AssuredIM managed fixed-maturity securities
Schedule of Investments [Line Items]
Long-term purchase commitment, amount		757
Equity in Earnings (Losses) of Investees | AGAS | AssuredIM managed fixed-maturity securities
Schedule of Investments [Line Items]
Investments, fair value disclosure	$ 549	$ 549		$ 543