Investments (Schedule of Other Investments) (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
Schedule of Investments [Line Items]
Other investments	$ 594,793				$ 549,805				$ 594,793	$ 549,805
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
Net unrealized gains recognized in earnings									24,737	11,542	(13,549)
Initial commitments	1,000,000								1,000,000
Fulfilled commitments	585,200								585,200
Unfunded commitments	429,900								429,900
Other Investments
Schedule of Investments [Line Items]
Net investment income									42,100	26,300	21,900
Net unrealized gains recognized in earnings due to change in estimate									1,900	(3,400)	(2,500)
Other Investments | Net investment income
Schedule of Investments [Line Items]
Net unrealized gains recognized in earnings									24,700	11,500	$ 10,400
Catastrophe bonds
Schedule of Investments [Line Items]
Other investments					335,209					335,209
Private equity investments
Schedule of Investments [Line Items]
Other investments					191,061					191,061
Senior secured bank loan funds
Schedule of Investments [Line Items]
Other investments					22,040					22,040
Hedge funds
Schedule of Investments [Line Items]
Other investments					1,495					1,495
Fair Value, Measurements, Recurring
Schedule of Investments [Line Items]
Other investments	594,793				549,805				594,793	549,805
Fair Value, Measurements, Recurring | Catastrophe bonds
Schedule of Investments [Line Items]
Other investments	380,475				335,209				380,475	335,209
Fair Value, Measurements, Recurring | Private equity investments
Schedule of Investments [Line Items]
Other investments	196,220				191,061				196,220	191,061
Fair Value, Measurements, Recurring | Senior secured bank loan funds
Schedule of Investments [Line Items]
Other investments	17,574				22,040				17,574	22,040
Fair Value, Measurements, Recurring | Hedge funds
Schedule of Investments [Line Items]
Other investments	$ 524				$ 1,495				$ 524	$ 1,495