Related Party Transactions and Major Customers (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
Related Party Transaction [Line Items]
Gross premiums written	$ 547,755	$ 625,677	$ 977,343	$ 1,159,652	$ 407,766	$ 640,269	$ 827,415	$ 922,090	$ 3,310,427	$ 2,797,540	$ 2,374,576
Gross premiums earned									2,779,796	2,307,219	1,874,993
Acquisition expenses	120,465	$ 109,761	$ 105,052	$ 97,711	98,598	$ 76,761	$ 88,251	$ 83,282	432,989	346,892	289,323
Net reinsurance costs and recoveries									(1,458,518)	(1,558,960)	(179,820)
Total gross reserve for claims and claim expenses	6,076,271				5,080,408				$ 6,076,271	$ 5,080,408	$ 2,848,294
Gross premiums written | Three Brokers
Related Party Transaction [Line Items]
Percent of gross premiums received									75.20%	76.40%	80.80%
Gross premiums written | AON
Related Party Transaction [Line Items]
Percent of gross premiums received									40.70%	42.80%	46.40%
Gross premiums written | Marsh
Related Party Transaction [Line Items]
Percent of gross premiums received									24.60%	23.80%	23.60%
Gross premiums written | Willis Towers Watson
Related Party Transaction [Line Items]
Percent of gross premiums received									9.90%	9.80%	10.80%
Tower Hill Companies
Related Party Transaction [Line Items]
Gross premiums written									$ 45,500	$ 39,100	$ 32,800
Gross premiums earned									43,800	35,700	32,300
Acquisition expenses									7,100	5,100	3,800
Related party receivables	19,300				14,100				19,300	14,100
Net reinsurance costs and recoveries									111,200	94,400	1,500
Total gross reserve for claims and claim expenses	$ 98,800				$ 65,300				98,800	65,300
Distributions received									12,100	8,300	9,000
Top Layer Reinsurance Ltd.
Related Party Transaction [Line Items]
Distributions received									4,000	20,000	0
Top Layer Reinsurance Ltd. | Management Fee
Related Party Transaction [Line Items]
Revenue recorded									$ 2,700	$ 2,700	$ 2,600