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Investments - Narrative (Details)		3 Months Ended		12 Months Ended
	Jul. 26, 2016 USD ($)	Mar. 31, 2017 USD ($) security	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) security	Jan. 01, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 02, 2012
Investment Securities [Line Items]
OTTI charge recognized		$ 300,000	$ 0
Catastrophe bonds		41,800,000				$ 42,500,000
Initial investment		2,300,000
Proceeds from Sale of Equity Method Investments		(9,300,000)	0
Equity Method Investment, Aggregate Cost		5,000,000	9,500,000	$ 9,500,000		12,100,000
Investment Funds		$ 0
Number Of Municipal Holdings | security		1		1
Municipal Holdings		$ 100,000
Aspen Bermuda Limited
Investment Securities [Line Items]
Transfer from Investments		0
MVI
Investment Securities [Line Items]
Initial investment		0
Proceeds from Sale of Equity Method Investments		0
Equity Method Investment, Aggregate Cost		500,000	800,000	$ 800,000		500,000
Chaspark
Investment Securities [Line Items]
Initial investment		0
Percentage of ownership acquired							58.50%
Proceeds from Sale of Equity Method Investments		(9,300,000)
Income (Loss) from Equity Method Investments		1,700,000	0
Equity Method Investment, Aggregate Cost		800,000	$ 8,700,000	$ 8,700,000		8,400,000
Bene
Investment Securities [Line Items]
Initial investment	$ 3,300,000	0
Percentage of ownership acquired	20.00%
Proceeds from Sale of Equity Method Investments		0
Equity Method Investment, Aggregate Cost		3,200,000				3,200,000
Digital Re
Investment Securities [Line Items]
Initial investment		2,300,000
Percentage of ownership acquired					49.00%
Proceeds from Sale of Equity Method Investments		0
Equity Method Investment, Aggregate Cost		$ 500,000			$ 2,300,000	$ 0

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Catastrophe Bond

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Investment Funds

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Investment Securities [Line Items]

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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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