Investments (Schedule of Other Investments) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Other investments	$ 1,087,377				$ 784,933				$ 1,087,377	$ 784,933
Net investment income	112,695	$ 113,844	$ 115,832	$ 81,462	53,338	$ 80,696	$ 71,356	$ 56,476	423,833	261,866	$ 222,209
Initial commitments	1,100,000								1,100,000
Fulfilled commitments	708,400								708,400
Unfunded commitments	411,300								411,300
Other investments
Schedule of Investments [Line Items]
Net investment income									54,200	23,000	42,100
Net unrealized gains recognized in earnings due to change in estimate									(5,500)	300	1,900
Other investments | Net investment income
Schedule of Investments [Line Items]
Net unrealized gains recognized in earnings									12,200	(8,300)	$ 24,700
Catastrophe bonds
Schedule of Investments [Line Items]
Other investments	781,641				516,571				781,641	516,571
Private equity investments
Schedule of Investments [Line Items]
Other investments	271,047				242,647				271,047	242,647
Senior secured bank loan funds
Schedule of Investments [Line Items]
Other investments	22,598				14,482				22,598	14,482
Hedge funds
Schedule of Investments [Line Items]
Other investments	$ 12,091				$ 11,233				$ 12,091	$ 11,233