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Investments - Additional Information (Narrative) (Details) - USD ($) |  |  | 1 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 18, 2017 | Jul. 26, 2016 | Jan. 31, 2015 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Sep. 19, 2018 | Sep. 18, 2018 | May 01, 2018 | Jan. 01, 2017 | Oct. 02, 2012
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Catastrophe bonds, trading, cost |  |  |  | $ 36,200,000 | $ 32,400,000 |  |  |  |  |  |
Restricted assets |  |  |  | 4,419,200,000 | 4,638,800,000 |  |  |  |  |  |
Other investments |  |  |  | 102,500,000 | 0 |  |  |  |  |  |
Investment in the period |  |  |  | 1,400,000 | 64,900,000 |  |  |  |  |  |
Proceeds from sale of equity method investments |  |  |  | 0 | (9,300,000) | $ 0 |  |  |  |  |
Equity method investment, aggregate cost |  |  |  | 67,100,000 | 66,400,000 | 12,100,000 |  |  |  |  |
Investment funded |  |  |  | 0 |  |  |  |  |  |  |
Other-than-temporary impairment charge |  |  |  | $ 0 | $ 700,000 | 0 |  |  |  |  |
BBB Emerging Market Debt |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of investment |  |  |  | 4.50% |  |  |  |  |  |  |
Real estate fund |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of investment |  |  |  | 1.30% |  |  |  |  |  |  |
Managed portfolio |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of investment |  |  |  | 5.80% | 10.00% |  |  |  |  |  |
Bene |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  | $ 3,300,000 |  | $ 1,200,000 | $ 0 |  |  |  |  |  |
Gross realized and unrealized gains (loss) |  |  |  | (900,000) | (300,000) | (100,000) |  |  |  |  |
Proceeds from sale of equity method investments |  |  |  |  | 0 |  |  |  |  |  |
Equity method investment, ownership percentage |  | 20.00% |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  | 3,200,000 | 2,900,000 | 3,200,000 |  |  |  |  |
Equity method investment, cash payment |  |  |  | 1,200,000 |  |  |  |  |  |  |
Chaspark |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  |  |  | 0 | 0 |  |  |  |  |  |
Percentage of ownership acquired |  |  |  |  |  |  |  |  |  |  | 58.50%
Gross realized and unrealized gains (loss) |  |  |  | 0 | 900,000 | 300,000 |  |  |  |  |
Proceeds from sale of equity method investments |  |  |  |  | (9,300,000) |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  | 0 | 0 | 8,400,000 |  |  |  |  |
MVI |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  |  | $ 800,000 | 200,000 | 100,000 |  |  |  |  |  |
Gross realized and unrealized gains (loss) |  |  |  | (200,000) | (100,000) | (300,000) |  |  |  |  |
Proceeds from sale of equity method investments |  |  |  |  | 0 |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  | 500,000 | 500,000 | 500,000 |  |  |  |  |
Digital Re |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  |  |  | 0 | 2,300,000 |  |  |  |  |  |
Gross realized and unrealized gains (loss) |  |  |  | 400,000 | 0 | 0 |  |  |  |  |
Proceeds from sale of equity method investments |  |  |  |  | 0 |  |  |  |  |  |
Equity method investment, ownership percentage |  |  |  |  |  |  |  |  |  | 49.00% |
Equity method investment, aggregate cost |  |  |  | 900,000 | 500,000 | 0 |  |  |  | $ 2,300,000 |
Crop Re |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  |  |  | 0 | 62,500,000 |  |  |  |  |  |
Proceeds from sale of equity method investments |  |  |  |  | 0 |  |  |  |  |  |
Equity method investment, ownership percentage | 23.20% |  |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost | $ 62,500,000 |  |  | 62,500,000 | 62,500,000 | $ 0 |  |  |  |  |
Equity method investment, cash payment | $ 5,900,000 |  |  |  |  |  |  |  |  |  |
AgriLogic Consulting, LLC |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage |  |  |  |  |  |  |  | 40.00% |  |  |
Equity method investment, aggregate cost |  |  |  |  |  |  |  | $ 0 |  |  |
CGB Insurance Diversified Services, Inc. | Crop Re |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Equity method investment, counterparty ownership percentage | 76.80% |  |  |  |  |  |  |  |  |  |
Limited Partner | Real estate fund |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Restricted assets |  |  |  | 0 | 100,000,000 |  |  |  |  |  |
Other investments |  |  |  | $ 102,500,000 | $ 0 |  | $ 13,800,000 |  | $ 86,200,000 |  |
AgriLogic Consulting, LLC | Consulting Relationships | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Sale of interest in consulting business |  |  |  |  |  |  |  | 60.00% |  |  |

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