Fair Value Measurement - Measured and Carried at Fair Value - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) source Security	Dec. 31, 2017 USD ($)
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Percentage of CDS contracts which are fair valued using minimum premium	18.00%	16.00%
Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets	$ 45	$ 48
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	98
Fixed maturity securities	$ 1,268
Equity Securities, FV-NI	$ 2	$ 2
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%
Total [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Number of sources of credit spread | source	3
Total [Member] | Recurring [Member] | Level 3 [Member] | Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount factor (as a percent)	2.34%	1.72%
Total [Member] | Recurring [Member] | Level 3 [Member] | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount factor (as a percent)	2.86%	2.55%