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Investments - Narrative (Details) - USD ($)		3 Months Ended		6 Months Ended		12 Months Ended
	Dec. 18, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	May 10, 2018	Mar. 31, 2018	Jan. 01, 2017	Jul. 26, 2016	Jan. 01, 2015
Investment Holdings [Line Items]
Catastrophe bonds		$ 35,500,000		$ 35,500,000		$ 32,400,000
Equity method investment, aggregate cost		67,100,000		67,100,000		66,400,000		$ 66,100,000
Restricted Assets		4,525,900,000		4,525,900,000		4,638,800,000
Other investments		86,200,000		86,200,000		0
Investment Funds		0		0
OTTI charge recognized		0	$ 100,000	0	$ 400,000
MVI
Investment Holdings [Line Items]
Equity method investment, aggregate cost		300,000		300,000		500,000		400,000			$ 800,000
Additional investment						100,000
Bene
Investment Holdings [Line Items]
Equity method investment, aggregate cost		3,800,000		3,800,000		2,900,000		2,700,000		$ 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%
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		$ 13,800,000		$ 13,800,000		$ 100,000,000
Other investments							$ 86,200,000

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Catastrophe Bond

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Equity Method Investment, Counterparty Ownership Percentage

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Investment Funds

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Restricted assets.

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This element represents the aggregate cost of investments 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 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of investments classified as other.

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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.

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