Investments and Cash - Narrative (Details) $ in Millions	1 Months Ended	12 Months Ended
	Feb. 28, 2017 USD ($)	Dec. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Investment [Line Items]
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		$ 97	$ 91
Number of outside managers managing investment portfolio		6
Number of securities with unrealized losses greater than 10% of book value for 12 months or more | Security		28	41
Total unrealized losses for securities having losses greater than 10% of book value for 12 months or more		$ 27	$ 59
Assets held-in-trust		269	285
Fair market value of company's pledged securities		18	116
Investments that were non-income producing during period		$ 0	$ 0
Investments [Member] | Available-for-sale Securities [Member]
Investment [Line Items]
Percentage of installment premiums denominated in currencies other than the U.S. dollar		99.20%
Investments [Member] | Internally Managed Portfolio [Member]
Investment [Line Items]
Percentage of installment premiums denominated in currencies other than the U.S. dollar		12.00%	15.00%
Assured Guaranty Subsidiaries [Member]
Investment [Line Items]
Assets held-in-trust		$ 1,677	$ 1,420
Future Equity Investments [Member]
Investment [Line Items]
Long-term Purchase Commitment, Amount	$ 100