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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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