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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Equity in earnings of other ventures	$ 5,874	$ 5,877	$ 6,812	$ 4,661	$ 4,143	$ 7,648	$ 5,826	$ 857	$ 23,224	$ 18,474	$ 8,030
Equity method investment, dividends and distributions									(36,500)	(26,100)	29,700
Undistributed earnings (losses) from equity method investments	$ (800)				$ 3,800				(800)	3,800	(6,300)
Tower Hill Companies
Schedule of Equity Method Investments [Line Items]
Equity in earnings of other ventures									10,337	9,605	(1,647)
Top Layer Re
Schedule of Equity Method Investments [Line Items]
Equity in earnings of other ventures									8,801	8,852	9,851
Other
Schedule of Equity Method Investments [Line Items]
Equity in earnings of other ventures									$ 4,086	$ 17	$ (174)