Fair Value Measurement - Narrative (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) source Security	Dec. 31, 2016 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Percentage of CDS contracts which are fair valued using minimum premium	16.00%	19.00%
Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets	$ 48	$ 52
Recurring [Member] \| Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Number of fixed maturity securities valued using model processes \| Security	93
Fixed maturity securities	$ 1,265
Available-for-Sale Debt Securities and Short Term Investments [Member] \| Recurring [Member] \| Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Percentage of fixed maturity securities valued using model processes to the Company's fixed-income securities and short-term investments at fair value	11.00%
Credit Default Swap [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Number of sources of credit spread \| source	3
Credit Default Swap [Member] \| Recurring [Member] \| Level 3 [Member] \| Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount factor (as a percent)	1.72%	1.00%
Credit Default Swap [Member] \| Recurring [Member] \| Level 3 [Member] \| Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount factor (as a percent)	2.55%	2.55%

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Represents the number of fixed maturity securities and short-term investments which have been valued using model processes set by the entity.

+ References

No definition available.

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Represents the number of sources on which credit spread is based.

+ References

No definition available.

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- Definition

This element represents the fair value investments categorized as other which are not in and of themselves material enough to require separate disclosure.

+ References

No definition available.

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- Definition

Represents the percentage of CDS contracts which are fair valued using minimum premium. Minimum premium is the minimum rate that the company would currently charge to assume similar risks.

+ References

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
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Line items represent financial 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.

+ References

No definition available.

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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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