Investments (Schedule of Other Investments) (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
Schedule of Investments [Line Items]
Other investments	$ 549,805				$ 481,621				$ 549,805	$ 481,621
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
Net unrealized gains (losses) recognized in earnings									11,542	(13,549)	(1,393)
Initial commitments	794,200								794,200
Fulfilled commitments	554,700								554,700
Unfunded commitments	249,400								249,400
Other Investments
Schedule of Investments [Line Items]
Net investment income									26,300	21,900	30,000
Net unrealized gains (losses) recognized in earnings due to change in estimate									(3,400)	(2,500)	(600)
Other Investments | Net investment income
Schedule of Investments [Line Items]
Net unrealized gains (losses) recognized in earnings									11,500	10,400	$ 17,700
Catastrophe bonds
Schedule of Investments [Line Items]
Other investments	335,209				241,253				335,209	241,253
Private equity partnerships
Schedule of Investments [Line Items]
Other investments	191,061				214,848				191,061	214,848
Senior secured bank loan funds
Schedule of Investments [Line Items]
Other investments	22,040				23,231				22,040	23,231
Hedge funds
Schedule of Investments [Line Items]
Other investments	$ 1,495				$ 2,289				$ 1,495	$ 2,289