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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Section S99
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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, from an acquisition.

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No definition available.

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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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Investments (Schedule of Investments in Other Ventures, under Equity Method) (Details) - USD ($) $ in Thousands		1 Months Ended			12 Months Ended
	Jul. 01, 2008	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 06, 1999
Schedule of Equity Method Investments [Line Items]
Investment					$ 143,775	$ 154,048
Investments in other ventures, under equity method					101,974	124,227
Equity method investee, additional contribution					0	0	$ 10,150
Tower Hill Companies
Schedule of Equity Method Investments [Line Items]
Investment					64,750	64,750
Investments in other ventures, under equity method					42,167	52,864
THIG
Schedule of Equity Method Investments [Line Items]
Investment	$ 50,000				$ 50,000	$ 50,000
Ownership %	25.00%				25.00%	25.00%
Investments in other ventures, under equity method					$ 20,856	$ 19,286
Intangible assets, net	$ 40,000
Goodwill	$ 7,800
Tower Hill
Schedule of Equity Method Investments [Line Items]
Investment					$ 10,000	$ 10,000
Ownership %					33.30%	32.30%
Investments in other ventures, under equity method					$ 14,917	$ 21,590
Tower Hill Re
Schedule of Equity Method Investments [Line Items]
Investment					$ 4,250	$ 4,250
Ownership %					25.00%	25.00%
Investments in other ventures, under equity method					$ 0	$ 2,903
Tower Hill Signature
Schedule of Equity Method Investments [Line Items]
Investment					$ 500	$ 500
Ownership %					25.00%	25.00%
Investments in other ventures, under equity method					$ 6,394	$ 9,085
Top Layer Re
Schedule of Equity Method Investments [Line Items]
Investment					$ 65,375	$ 65,375		$ 13,100
Ownership %					50.00%	50.00%		50.00%
Investments in other ventures, under equity method					$ 50,211	$ 60,360
Equity method investee, additional contribution		$ 18,000	$ 20,500	$ 13,800
Other
Schedule of Equity Method Investments [Line Items]
Investment					$ 13,650	$ 23,923
Ownership %					40.40%	41.80%
Investments in other ventures, under equity method					$ 9,596	$ 11,003