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Investments - Narrative (Details) - USD ($) |  | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 18, 2017 | Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | Jan. 01, 2017 | Jul. 26, 2016 | Jan. 01, 2015
Investment Holdings [Line Items] |  |  |  |  |  |  |
Catastrophe bonds |  | $ 34,800,000 |  | $ 32,400,000 |  |  |
Equity method investment, aggregate cost |  | 66,100,000 |  | 66,400,000 |  |  |
Distribution received |  | 0 | $ (9,300,000) |  |  |  |
Investment Funds |  | 0 |  |  |  |  |
OTTI charge recognized |  | 0 | 300,000 |  |  |  |
MVI |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |
Equity method investment, aggregate cost |  | 400,000 |  | 500,000 |  |  | $ 800,000
Additional investment |  |  |  | 100,000 |  |  |
Change in fair value of investment |  | (100,000) | 0 |  |  |  |
Chaspark |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |
Change in fair value of investment |  | 0 | 1,700,000 |  |  |  |
Bene |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |
Equity method investment, aggregate cost |  | 2,700,000 |  | 2,900,000 |  | $ 3,300,000 |
Equity method investment, ownership percentage |  |  |  |  |  | 20.00% |
Change in fair value of investment |  | (200,000) | $ 0 |  |  |  |
Digital Re |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |
Equity method investment, aggregate cost |  | 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 |  |  |
Additional investment | $ 5,900,000 |  |  |  |  |  |
Equity method investment, ownership percentage | 23.20% |  |  |  |  |  |
CGB DS | Crop Re |  |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |  |
Equity method investment, counterparty ownership Percentage | 76.80% |  |  |  |  |  |

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- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571 + Details
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- Definition The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e25351-111560 + Details
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- Definition This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(12)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(10)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(f)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
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- Definition The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585 + Details
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