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Investments - Narrative (Details) |  | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jul. 26, 2016 USD ($) | Mar. 31, 2017 USD ($) security | Mar. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) security | Jan. 01, 2017 USD ($) | Dec. 31, 2016 USD ($) | Oct. 02, 2012
Investment Securities [Line Items] |  |  |  |  |  |  |
OTTI charge recognized |  | $ 300,000 | $ 0 |  |  |  |
Catastrophe bonds |  | 41,800,000 |  |  |  | $ 42,500,000 |
Initial investment |  | 2,300,000 |  |  |  |  |
Proceeds from Sale of Equity Method Investments |  | (9,300,000) | 0 |  |  |  |
Equity Method Investment, Aggregate Cost |  | 5,000,000 | 9,500,000 | $ 9,500,000 |  | 12,100,000 |
Investment Funds |  | $ 0 |  |  |  |  |
Number Of Municipal Holdings | security |  | 1 |  | 1 |  |  |
Municipal Holdings |  | $ 100,000 |  |  |  |  |
Aspen Bermuda Limited |  |  |  |  |  |  |
Investment Securities [Line Items] |  |  |  |  |  |  |
Transfer from Investments |  | 0 |  |  |  |  |
MVI |  |  |  |  |  |  |
Investment Securities [Line Items] |  |  |  |  |  |  |
Initial investment |  | 0 |  |  |  |  |
Proceeds from Sale of Equity Method Investments |  | 0 |  |  |  |  |
Equity Method Investment, Aggregate Cost |  | 500,000 | 800,000 | $ 800,000 |  | 500,000 |
Chaspark |  |  |  |  |  |  |
Investment Securities [Line Items] |  |  |  |  |  |  |
Initial investment |  | 0 |  |  |  |  |
Percentage of ownership acquired |  |  |  |  |  |  | 58.50%
Proceeds from Sale of Equity Method Investments |  | (9,300,000) |  |  |  |  |
Income (Loss) from Equity Method Investments |  | 1,700,000 | 0 |  |  |  |
Equity Method Investment, Aggregate Cost |  | 800,000 | $ 8,700,000 | $ 8,700,000 |  | 8,400,000 |
Bene |  |  |  |  |  |  |
Investment Securities [Line Items] |  |  |  |  |  |  |
Initial investment | $ 3,300,000 | 0 |  |  |  |  |
Percentage of ownership acquired | 20.00% |  |  |  |  |  |
Proceeds from Sale of Equity Method Investments |  | 0 |  |  |  |  |
Equity Method Investment, Aggregate Cost |  | 3,200,000 |  |  |  | 3,200,000 |
Digital Re |  |  |  |  |  |  |
Investment Securities [Line Items] |  |  |  |  |  |  |
Initial investment |  | 2,300,000 |  |  |  |  |
Percentage of ownership acquired |  |  |  |  | 49.00% |  |
Proceeds from Sale of Equity Method Investments |  | 0 |  |  |  |  |
Equity Method Investment, Aggregate Cost |  | $ 500,000 |  |  | $ 2,300,000 | $ 0 |

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