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Investments - Narrative (Details) | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Dec. 31, 2016 USD ($) security | Jun. 30, 2017 USD ($) | Jan. 01, 2017 USD ($) | Jul. 26, 2016 USD ($) | Jun. 30, 2016 USD ($) | Dec. 31, 2015 USD ($) | Jan. 01, 2015 USD ($) | Oct. 02, 2012
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
OTTI charge recognized | $ 100,000 | $ 0 | $ 500,000 | $ 0 |  |  |  |  |  |  |  |
Catastrophe bonds | 30,300,000 |  | 30,300,000 |  | $ 42,500,000 |  |  |  |  |  |  |
Equity Method Investment, Aggregate Cost | 4,000,000 | 13,000,000 | 4,000,000 | 13,000,000 | $ 12,100,000 | $ 3,900,000 |  |  | $ 8,700,000 | $ 8,900,000 |  |
Additional investment | 100,000 |  | 100,000 |  |  |  |  |  |  |  |  |
Distribution received |  |  | (9,300,000) | 0 |  |  |  |  |  |  |  |
Investment Funds | 0 |  | 0 |  |  |  |  |  |  |  |  |
Number Of Municipal Holdings | security |  |  |  |  | 1 |  |  |  |  |  |  |
MVI |  |  |  |  |  |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Equity Method Investment, Aggregate Cost | 500,000 | 600,000 | 500,000 | 600,000 | $ 500,000 | 400,000 |  |  | 600,000 | 800,000 | $ 800,000 |
Additional investment | 100,000 |  | 100,000 |  |  |  |  |  |  |  |  |
Change in fair value of investment | 0 | 0 | (100,000) | 0 |  |  |  |  |  |  |  |
Distribution received |  |  | 0 |  |  |  |  |  |  |  |  |
Chaspark |  |  |  |  |  |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Equity Method Investment, Aggregate Cost | 0 | 9,100,000 | 0 | 9,100,000 | 8,400,000 | 0 |  |  | 8,100,000 | $ 8,100,000 |  |
Additional investment | 0 |  | 0 |  |  |  |  |  |  |  |  |
Equity Method Investment, Ownership Percentage |  |  |  |  |  |  |  |  |  |  |  | 58.50%
Change in fair value of investment | 0 | 1,000,000 | 900,000 | 1,000,000 |  |  |  |  |  |  |  |
Distribution received |  |  | (9,300,000) |  |  |  |  |  |  |  |  |
Bene |  |  |  |  |  |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Equity Method Investment, Aggregate Cost | 3,000,000 | 3,300,000 | 3,000,000 | 3,300,000 | 3,200,000 | 3,000,000 |  | $ 3,300,000 | $ 0 |  |  |
Additional investment | 0 |  | 0 |  |  |  |  |  |  |  |  |
Equity Method Investment, Ownership Percentage |  |  |  |  |  |  |  | 20.00% |  |  |  |
Change in fair value of investment | 0 | $ 0 | (200,000) | $ 0 |  |  |  |  |  |  |  |
Distribution received |  |  | 0 |  |  |  |  |  |  |  |  |
Digital Re |  |  |  |  |  |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Equity Method Investment, Aggregate Cost | 500,000 |  | 500,000 |  | $ 0 | $ 500,000 | $ 2,300,000 |  |  |  |  |
Additional investment | $ 0 |  | 0 |  |  |  |  |  |  |  |  |
Equity Method Investment, Ownership Percentage |  |  |  |  |  |  | 49.00% |  |  |  |  |
Distribution received |  |  | $ 0 |  |  |  |  |  |  |  |  |

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