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Investments - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) security	Jun. 30, 2017 USD ($)	Jan. 01, 2017 USD ($)	Jul. 26, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 01, 2015 USD ($)	Oct. 02, 2012
Investment Holdings [Line Items]
OTTI charge recognized	$ 100,000	$ 0	$ 500,000	$ 0
Catastrophe bonds	30,300,000		30,300,000		$ 42,500,000
Equity Method Investment, Aggregate Cost	4,000,000	13,000,000	4,000,000	13,000,000	$ 12,100,000	$ 3,900,000			$ 8,700,000	$ 8,900,000
Additional investment	100,000		100,000
Distribution received			(9,300,000)	0
Investment Funds	0		0
Number Of Municipal Holdings | security					1
MVI
Investment Holdings [Line Items]
Equity Method Investment, Aggregate Cost	500,000	600,000	500,000	600,000	$ 500,000	400,000			600,000	800,000	$ 800,000
Additional investment	100,000		100,000
Change in fair value of investment	0	0	(100,000)	0
Distribution received			0
Chaspark
Investment Holdings [Line Items]
Equity Method Investment, Aggregate Cost	0	9,100,000	0	9,100,000	8,400,000	0			8,100,000	$ 8,100,000
Additional investment	0		0
Equity Method Investment, Ownership Percentage												58.50%
Change in fair value of investment	0	1,000,000	900,000	1,000,000
Distribution received			(9,300,000)
Bene
Investment Holdings [Line Items]
Equity Method Investment, Aggregate Cost	3,000,000	3,300,000	3,000,000	3,300,000	3,200,000	3,000,000		$ 3,300,000	$ 0
Additional investment	0		0
Equity Method Investment, Ownership Percentage								20.00%
Change in fair value of investment	0	$ 0	(200,000)	$ 0
Distribution received			0
Digital Re
Investment Holdings [Line Items]
Equity Method Investment, Aggregate Cost	500,000		500,000		$ 0	$ 500,000	$ 2,300,000
Additional investment	$ 0		0
Equity Method Investment, Ownership Percentage							49.00%
Distribution received			$ 0

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Catastrophe Bond

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Investment Funds

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Number of municipal holdings.

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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
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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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