Investments and Cash - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) Security	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) Security manager	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) Security
Schedule of Investments [Line Items]
Number of outside managers managing investment portfolio | manager			3
Accrued investment income	$ 79,000,000		$ 79,000,000		$ 79,000,000
Number of securities with unrealized losses greater than 10% of book value for 12 months or more | Security	22		22		19
Total unrealized losses for securities having losses greater than 10% of book value for 12 months or more	$ 13,000,000		$ 13,000,000		$ 25,000,000
Assets held-in-trust	290,000,000		290,000,000		280,000,000
Proceeds from sale of fixed-maturity securities available-for-sale	137,000,000	$ 405,000,000	627,000,000	$ 1,306,000,000
Limited Liability Company
Schedule of Investments [Line Items]
Equity method investments, fair value	58,000,000		58,000,000
Future Equity Investments
Schedule of Investments [Line Items]
Remaining minimum amount committed	84,000,000		84,000,000
AGL Subsidiaries
Schedule of Investments [Line Items]
Assets held-in-trust	1,457,000,000		1,457,000,000		1,502,000,000
Maximum | Future Equity Investments
Schedule of Investments [Line Items]
Long-term purchase commitment, amount			100,000,000
AssuredIM | AGAS
Schedule of Investments [Line Items]
Undrawn committed capital	148,000,000		148,000,000
AssuredIM | Maximum | AGL Subsidiaries | Equity Method Investments
Schedule of Investments [Line Items]
Long-term purchase commitment, amount					79,000,000
Long-term purchase commitment fair value					$ 77,000,000
AssuredIM | Maximum | AGAS | Future Equity Investments
Schedule of Investments [Line Items]
Long-term purchase commitment, amount			500,000,000
AssuredIM | Maximum | AGAS | Equity Method Investments
Schedule of Investments [Line Items]
Long-term purchase commitment, amount			352,000,000
Long-term purchase commitment fair value	$ 379,000,000		379,000,000
Various Future Consolidated Assured Investment Management Funds | Maximum | AGAS | Future Equity Investments
Schedule of Investments [Line Items]
Long-term purchase commitment, amount			106,000,000
Individually Managed Accounts with AssuredIM | AGL Subsidiaries | Municipal Obligations
Schedule of Investments [Line Items]
Long-term purchase commitment, amount			250,000,000
Individually Managed Accounts with AssuredIM | AGL Subsidiaries | Collateralized Loan Obligations
Schedule of Investments [Line Items]
Long-term purchase commitment, amount			263,000,000
Long-term purchase commitment, additional authorized amount			$ 37,000,000