Investments (Schedule of Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income before investment expenses	$ 59,799	$ 58,017
Investment expenses	(3,323)	(3,692)
Net investment income	56,476	54,325
Fixed maturity investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income before investment expenses	45,643	43,419
Short term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income before investment expenses	5,304	1,724
Equity investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income before investment expenses	698	811
Private equity investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income before investment expenses	(434)	7,802
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income before investment expenses	8,023	4,072
Cash and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income before investment expenses	$ 565	$ 189