Represents the number of securities which had unrealized losses greater than 10% of book value for 12 months or more.

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Represents the continuous period of decline in the market value of a security by any percentage below the amortized cost considered for the assessment of the impairment of investments.

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Managers Managing Investment Portfolio, Investment Guidelines, Percent of Externally Managed Portfolio

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Represents the minimum percentage of decline in the market value of securities below the amortized cost considered for the assessment of the impairment of investments.

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Non-Income Producing Investments

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Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The total amount of cash and securities held by third party trustees pursuant to terms of debt instruments or other agreements as of the date of each statement of financial position presented, which can be used by the trustee only to pay the noncurrent portion of specified obligations.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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The difference between the carrying value and the sale price of equity securities, not separately or otherwise categorized as trading or available-for-sale. This element includes investments in which the entity holds a small ownership stake (generally, less than 20% of the shares outstanding) and cannot exert significant influence.

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The minimum amount the entity agreed to spend under the long-term purchase commitment.

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Investments and Cash - Narrative (Details) $ in Millions	1 Months Ended	12 Months Ended
Investments and Cash - Narrative (Details) $ in Millions	Feb. 28, 2017 USD ($)	Dec. 31, 2018 USD ($) Security manager	Dec. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($)
Investment [Line Items]
Number of Outside Managers Managing Investment Portfolio \| manager		7
Sales		$ 1,180	$ 1,701	$ 1,365
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
Effect of adoption of ASU 2016-01			(32)
Accrued investment income		$ 91	$ 97
Number of securities with unrealized losses greater than 10% of book value for 12 months or more \| Security		38	28
Total unrealized losses for securities having losses greater than 10% of book value for 12 months or more		$ 43	$ 27
Assets held-in-trust		266	287
Investments that were non-income producing during period		0	0
Other [Member]
Investment [Line Items]
Gain (Loss) on Sale of Equity Investments		$ 31
Investments [Member] \| Available-for-sale Securities [Member]
Investment [Line Items]
Percentage of installment premiums denominated in currencies other than the U.S. dollar		99.50%
Assured Guaranty Subsidiaries [Member]
Investment [Line Items]
Assets held-in-trust		$ 1,855	1,677
Other Income [Member]
Investment [Line Items]
Equity Securities, FV-NI, Realized Gain (Loss)		$ 27
Accounting Standards Update 2016-01 [Member] \| Retained Earnings [Member] \| Redeemable Preferred Stock [Member]
Investment [Line Items]
Effect of adoption of ASU 2016-01			$ (1)
Maximum [Member] \| Standard & Poor's, A- Rating [Member] \| Moody's, A3 Rating [Member]
Investment [Line Items]
Managers Managing Investment Portfolio, Investment Guidelines, Percent of Externally Managed Portfolio		5.00%
Future Equity Investments [Member]
Investment [Line Items]
Long-term purchase commitment, amount	$ 100
Purchase Commitment, Remaining Minimum Amount Committed		$ 83