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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Equity in (losses) earnings of other ventures	$ 2,200	$ 1,794	$ 5,543	$ (1,507)	$ 4,960	$ (11,630)	$ 6,022	$ 1,611	$ 8,030	$ 963	$ 20,481
Equity method investment, dividends or distributions									29,700	9,400	13,300
Undistributed losses from equity method investments	$ (6,300)				$ (5,500)				(6,300)	(5,500)
Top Layer Re
Schedule of Equity Method Investments [Line Items]
Equity in (losses) earnings of other ventures									9,851	(8,576)	8,026
Tower Hill Companies
Schedule of Equity Method Investments [Line Items]
Equity in (losses) earnings of other ventures									(1,647)	10,379	13,116
Other
Schedule of Equity Method Investments [Line Items]
Equity in (losses) earnings of other ventures									$ (174)	$ (840)	$ (661)