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Investments - Narrative (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) security	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) security	Mar. 31, 2017 USD ($)	Jan. 01, 2017 USD ($)	Jul. 26, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Oct. 02, 2012
Investment Holdings [Line Items]
OTTI charge recognized	$ 100,000	$ 0	$ 400,000	$ 0
Catastrophe bonds	28,300,000		28,300,000		$ 42,500,000
Equity Method Investment, Aggregate Cost	3,900,000	8,700,000	3,900,000	8,700,000	$ 12,100,000	$ 5,000,000			$ 8,900,000	$ 8,900,000
Distribution received			(9,300,000)	0
Investment Funds	0		$ 0
Number Of Municipal Holdings | security			1		1
Municipal Holdings			$ 100,000
MVI
Investment Holdings [Line Items]
Equity Method Investment, Aggregate Cost	400,000	600,000	400,000	600,000	$ 500,000	500,000			800,000	800,000
Income (Loss) from Equity Method Investments	(100,000)	0	(100,000)	0
Distribution received			0
Chaspark
Investment Holdings [Line Items]
Equity Method Investment, Aggregate Cost	0	8,100,000	0	8,100,000	8,400,000	800,000			$ 8,100,000	$ 8,100,000
Equity Method Investment, Ownership Percentage											58.50%
Income (Loss) from Equity Method Investments	(800,000)	0	900,000	0
Distribution received			(9,300,000)
Bene
Investment Holdings [Line Items]
Equity Method Investment, Aggregate Cost	3,000,000		3,000,000		3,200,000	3,200,000		$ 3,300,000
Equity Method Investment, Ownership Percentage								20.00%
Income (Loss) from Equity Method Investments	(200,000)	$ 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
Equity Method Investment, Ownership Percentage							49.00%
Distribution received			$ 0

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- Definition

Catastrophe Bond

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Investment Funds

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Municipal Holdings

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This element represents the aggregate cost of investments accounted for under the equity method of accounting.

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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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