Investments (Schedule of Net Investment Income) (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
Net Investment Income [Line Items]
Gross investment income									$ 441,478	$ 276,780	$ 237,596
Investment expenses									(17,645)	(14,914)	(15,387)
Net investment income	$ 112,695	$ 113,844	$ 115,832	$ 81,462	$ 53,338	$ 80,696	$ 71,356	$ 56,476	423,833	261,866	222,209
Fixed maturity investments
Net Investment Income [Line Items]
Gross investment income									318,503	211,973	179,624
Short term investments
Net Investment Income [Line Items]
Gross investment income									56,264	33,571	11,082
Equity investments
Net Investment Income [Line Items]
Gross investment income									4,808	4,474	3,628
Private equity investments
Net Investment Income [Line Items]
Gross investment income									14,981	477	33,999
Other
Net Investment Income [Line Items]
Gross investment income									39,246	22,475	8,067
Cash and cash equivalents
Net Investment Income [Line Items]
Gross investment income									$ 7,676	$ 3,810	$ 1,196