Investments and Cash - Narrative (Details)	1 Months Ended	12 Months Ended
	Feb. 28, 2017 USD ($)	Dec. 31, 2019 USD ($) Security manager Investment_Funds	Dec. 31, 2018 USD ($) Security	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)
Investment [Line Items]
Number of investment portfolio managers | manager		6
Sales		$ 1,805,000,000	$ 1,180,000,000	$ 1,701,000,000
Percentage of decline in market value of security below amortized cost, considered for assessing impairment of investments		20.00%
Continuous period of decline in market value below amortized costs considered for assessing impairment of investments		6 months
Continuous period of decline in market value of security, considered for assessing impairment of investments		12 months
Accrued investment income		$ 79,000,000	$ 91,000,000
Number of securities with unrealized losses greater than 10% of book value for 12 months or more | Security		19	38
Total unrealized losses for securities having losses greater than 10% of book value for 12 months or more		$ 25,000,000	$ 43,000,000
Assets held-in-trust		280,000,000	266,000,000
Investments that were non-income producing during period		$ 0	0
Other [Member]
Investment [Line Items]
Gain (Loss) on Sale of Investments			31,000,000
Investments [Member] | Available-for-sale Securities [Member]
Investment [Line Items]
Percentage of installment premiums denominated in currencies other than the U.S. dollar		98.80%
Assured Guaranty Subsidiaries [Member]
Investment [Line Items]
Assets held-in-trust		$ 1,502,000,000	1,855,000,000
Other Income [Member]
Investment [Line Items]
Equity securities, realized gain (loss)		0	$ 27,000,000
Accounting Standards Update 2016-01 [Member]
Investment [Line Items]
Effect of adoption of ASU 2016-01					$ (32,000,000)
Future Equity Investments [Member]
Investment [Line Items]
Purchase commitment, remaining minimum amount committed		86,000,000
Future Equity Investments [Member] | Assured Guaranty Subsidiaries [Member] | Assured IM Funds [Member]
Investment [Line Items]
Purchase commitment, remaining minimum amount committed		422,000,000
Future Equity Investments [Member] | Maximum [Member]
Investment [Line Items]
Long-term purchase commitment, amount	$ 100,000,000
Future Equity Investments [Member] | Maximum [Member] | Assured Guaranty Subsidiaries [Member] | Assured IM Funds [Member]
Investment [Line Items]
Long-term purchase commitment, amount		500,000,000
Future Equity Investments [Member] | Maximum [Member] | Assured Guaranty Subsidiaries [Member] | Various Future Consolidated Assured IM Funds [Member]
Investment [Line Items]
Long-term purchase commitment, amount		$ 114,000,000
Number of Investment Funds | Investment_Funds		3
Equity Method Investments [Member] | Maximum [Member] | Assured Guaranty Subsidiaries [Member] | Assured IM Funds [Member]
Investment [Line Items]
Long-term purchase commitment, amount		$ 79,000,000