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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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Catastrophe Bond

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Equity Method Investment, Counterparty Ownership Percentage

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+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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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.

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