Investments (Schedule of Net Investment Income) (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
Net Investment Income [Line Items]
Gross investment income									$ 237,596	$ 197,147	$ 166,767
Investment expenses									(15,387)	(15,421)	(14,200)
Net investment income	$ 73,464	$ 40,257	$ 54,163	$ 54,325	$ 47,316	$ 51,423	$ 54,124	$ 28,863	222,209	181,726	152,567
Fixed maturity investments
Net Investment Income [Line Items]
Gross investment income									179,624	160,661	134,800
Short term investments
Net Investment Income [Line Items]
Gross investment income									11,082	5,127	1,227
Equity investments
Net Investment Income [Line Items]
Gross investment income									3,628	4,235	8,346
Private equity investments
Net Investment Income [Line Items]
Gross investment income									33,999	6,155	9,455
Other
Net Investment Income [Line Items]
Gross investment income									8,067	20,181	12,472
Cash and cash equivalents
Net Investment Income [Line Items]
Gross investment income									$ 1,196	$ 788	$ 467