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Basis of Preparation and Significant Accounting Policies - Additional Information (Details) $ in Millions		12 Months Ended
		Dec. 31, 2017 USD ($) period Investment	Dec. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Threshold Period of Value Decline in Equity Securities to be considered Other than Temporary Impairment		12 months
Non-Payment Of Dividends, Number Of Periods | period		6
Number of investments in VIEs | Investment		2	3
Computer Equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		3 years
Computer Equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		5 years
Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		4 years
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		15 years
Fixed maturities
Property, Plant and Equipment [Line Items]
Percentage of Value Decline in Securities to be considered Other than Temporary Impairment		20.00%
Accounting Standards Update 2016-09 | Retained earnings
Property, Plant and Equipment [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[1]	$ 2.8	$ 0.0	$ 0.0
Accounting Standards Update 2016-09 | Additional paid-in capital
Property, Plant and Equipment [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 7.9
[1] The $2.8 million relates to the cumulative effect-adjustment to opening retained earnings as a result of the classification of restricted share units as equity following the adoption of ASU 2016-09. The adjustment has been applied using a modified retrospective approach.

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