Investments (Schedule of Net Investment Income) (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
Net Investment Income [Line Items]
Gross investment income									$ 276,780	$ 237,596	$ 197,147
Investment expenses									(14,914)	(15,387)	(15,421)
Net investment income	$ 53,338	$ 80,696	$ 71,356	$ 56,476	$ 73,464	$ 40,257	$ 54,163	$ 54,325	261,866	222,209	181,726
Fixed maturity investments
Net Investment Income [Line Items]
Gross investment income									211,973	179,624	160,661
Short term investments
Net Investment Income [Line Items]
Gross investment income									33,571	11,082	5,127
Equity investments
Net Investment Income [Line Items]
Gross investment income									4,474	3,628	4,235
Private equity investments
Net Investment Income [Line Items]
Gross investment income									477	33,999	6,155
Other
Net Investment Income [Line Items]
Gross investment income									22,475	8,067	20,181
Cash and cash equivalents
Net Investment Income [Line Items]
Gross investment income									$ 3,810	$ 1,196	$ 788