Segment Reporting (Schedule of Gross Premiums Written Allocated to the Territory of Coverage Exposure) (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
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 905,479	$ 861,068	$ 1,476,908	$ 1,564,295	$ 547,755	$ 625,677	$ 977,343	$ 1,159,652	$ 4,807,750	$ 3,310,427	$ 2,797,540
Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									0	0	(7)
Property
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									2,430,985	1,760,926	1,440,437
Property | U.S. and Caribbean
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									1,368,205	978,063	954,269
Property | Worldwide
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									643,744	464,311	305,915
Property | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									182,544	144,857	49,486
Property | Japan
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									90,328	71,601	49,821
Property | Worldwide (excluding U.S.)
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									79,393	66,872	48,182
Property | Australia and New Zealand
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									32,203	19,273	14,151
Property | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									34,568	15,949	18,613
Casualty and Specialty
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									2,376,765	1,549,501	1,357,110
Casualty and Specialty | U.S. and Caribbean
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									1,071,170	667,125	622,757
Casualty and Specialty | Worldwide
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									935,626	776,976	686,253
Casualty and Specialty | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									227,178	15,296	9,752
Casualty and Specialty | Worldwide (excluding U.S.)
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									25,291	31,734	10,104
Casualty and Specialty | Australia and New Zealand
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									34,053	3,667	4,141
Casualty and Specialty | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									$ 83,447	$ 54,703	$ 24,103