Investments (Schedule of Other Investments) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Other investments	$ 784,933				$ 594,793				$ 784,933	$ 594,793
Net investment income	53,338	$ 80,696	$ 71,356	$ 56,476	73,464	$ 40,257	$ 54,163	$ 54,325	261,866	222,209	$ 181,726
Net unrealized gains recognized in earnings									(8,309)	24,737	11,542
Initial commitments	1,200,000								1,200,000
Fulfilled commitments	686,400								686,400
Unfunded commitments	470,800								470,800
Other investments
Schedule of Investments [Line Items]
Net investment income									23,000	42,100	26,300
Net unrealized gains recognized in earnings due to change in estimate									300	1,900	(3,400)
Other investments | Net investment income
Schedule of Investments [Line Items]
Net unrealized gains recognized in earnings									(8,300)	24,700	$ 11,500
Catastrophe bonds
Schedule of Investments [Line Items]
Other investments	516,571				380,475				516,571	380,475
Private equity investments
Schedule of Investments [Line Items]
Other investments	242,647				196,220				242,647	196,220
Senior secured bank loan funds
Schedule of Investments [Line Items]
Other investments	14,482				17,574				14,482	17,574
Hedge funds
Schedule of Investments [Line Items]
Other investments	$ 11,233				$ 524				$ 11,233	$ 524