Investments (Schedule of Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Investment Income [Line Items]
Gross investment income									$ 197,147	$ 166,767	$ 135,655
Investment expenses									(15,421)	(14,200)	(11,339)
Net investment income	$ 47,316	$ 51,423	$ 54,124	$ 28,863	$ 45,918	$ 28,338	$ 38,604	$ 39,707	181,726	152,567	124,316
Fixed maturity investments
Net Investment Income [Line Items]
Gross investment income									160,661	134,800	100,855
Short term investments
Net Investment Income [Line Items]
Gross investment income									5,127	1,227	944
Equity investments
Net Investment Income [Line Items]
Gross investment income									4,235	8,346	3,450
Private equity investments
Net Investment Income [Line Items]
Gross investment income									6,155	9,455	18,974
Other
Net Investment Income [Line Items]
Gross investment income									20,181	12,472	11,037
Cash and cash equivalents
Net Investment Income [Line Items]
Gross investment income									$ 788	$ 467	$ 395