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Investments - Narrative (Details) - USD ($) |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 18, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Sep. 19, 2018 | Sep. 18, 2018 | Jun. 30, 2018 | May 10, 2018 | Jan. 01, 2017 | Jul. 26, 2016 | Jan. 01, 2015
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Catastrophe bonds |  | $ 37,400,000 |  | $ 37,400,000 |  | $ 32,400,000 |  |  |  |  |  |  |
Equity method investment, aggregate cost |  | 66,900,000 |  | 66,900,000 |  | 66,400,000 |  |  | $ 67,100,000 |  |  |  |
Restricted assets |  | 4,526,400,000 |  | 4,526,400,000 |  | 4,638,800,000 |  |  |  |  |  |  |
Other investments |  | 100,500,000 |  | 100,500,000 |  | 0 |  |  |  |  |  |  |
Investment funds |  | 0 |  | 0 |  | 0 |  |  |  |  |  |  |
OTTI charge recognized |  | 0 | $ 100,000 | 0 | $ 500,000 |  |  |  |  |  |  |  |
MVI |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  | 400,000 |  | 400,000 |  | 500,000 |  |  | 300,000 |  |  |  | $ 800,000
Additional investment |  |  |  | 100,000 |  | 100,000 |  |  |  |  |  |  |
Bene |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  | 3,500,000 |  | 3,500,000 |  | 2,900,000 |  |  | 3,800,000 |  |  | $ 3,300,000 |
Additional investment |  |  |  |  |  | 1,200,000 |  |  |  |  |  |  |
Equity method investment, ownership percentage |  |  |  |  |  |  |  |  |  |  |  | 20.00% |
Digital Re |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  | 500,000 |  | 500,000 |  | 500,000 |  |  | 500,000 |  | $ 2,300,000 |  |
Equity method investment, ownership percentage |  |  |  |  |  |  |  |  |  |  | 49.00% |  |
Crop Re |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost | $ 62,500,000 | 62,500,000 |  | 62,500,000 |  | 62,500,000 |  |  | $ 62,500,000 |  |  |  |
Additional investment | $ 5,900,000 |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage | 23.20% |  |  |  |  |  |  |  |  |  |  |  |
AgriLogic Consulting, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  |  |  |  |  | $ 0 |  |  |  |  |
Equity method investment, ownership percentage |  |  |  |  |  |  |  | 40.00% |  |  |  |  |
CGB DS | Crop Re |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, counterparty ownership percentage | 76.80% |  |  |  |  |  |  |  |  |  |  |  |
Limited Partner | Real estate funds |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted assets |  | $ 0 |  | $ 0 |  | $ 100,000,000 |  |  |  |  |  |  |
Other investments |  |  |  |  |  |  | $ 13,800,000 |  |  | $ 86,200,000 |  |  |
Disposed of by Sale | Consulting Relationships | AgriLogic Consulting, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Sale of interest of consulting business |  |  |  |  |  |  |  | 60.00% |  |  |  |  |

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