Investments (Schedule of Equity In (Losses) Earnings Of Other Ventures, Under Equity Method) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Equity in earnings (losses) of other ventures	$ 4,960	$ (11,630)	$ 6,022	$ 1,611	$ 3,296	$ 5,730	$ 6,160	$ 5,295	$ 963	$ 20,481	$ 26,075
Equity method investment, dividends or distributions									9,400	13,300	10,300
Undistributed earnings (losses) from equity method investments	$ (5,500)				$ 10,100				(5,500)	10,100
Tower Hill Companies
Schedule of Equity Method Investments [Line Items]
Equity in earnings (losses) of other ventures									10,379	13,116	18,376
Top Layer Reinsurance Ltd.
Schedule of Equity Method Investments [Line Items]
Equity in earnings (losses) of other ventures									(8,576)	8,026	10,411
Other
Schedule of Equity Method Investments [Line Items]
Equity in earnings (losses) of other ventures									$ (840)	$ (661)	$ (2,712)