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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity method investment, dividends or distributions									$ (26,100)	$ 29,700	$ 9,400
Equity in earnings (losses) of other ventures	$ 4,143	$ 7,648	$ 5,826	$ 857	$ 2,200	$ 1,794	$ 5,543	$ (1,507)	18,474	8,030	963
Undistributed earnings (losses) from equity method investments	$ (3,800)				$ (6,300)				(3,800)	(6,300)
Top Layer Re
Schedule of Equity Method Investments [Line Items]
Equity in earnings (losses) of other ventures									8,852	9,851	(8,576)
Tower Hill Companies
Schedule of Equity Method Investments [Line Items]
Equity in earnings (losses) of other ventures									9,605	(1,647)	10,379
Other
Schedule of Equity Method Investments [Line Items]
Equity in earnings (losses) of other ventures									$ 17	$ (174)	$ (840)