Related Party Transactions and Major Customers (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Gross premiums written	$ 407,766	$ 640,269	$ 827,415	$ 922,090	$ 323,091	$ 430,224	$ 759,128	$ 862,133	$ 2,797,540	$ 2,374,576	$ 2,011,310
Gross premiums earned									2,307,219	1,874,993	1,769,088
Acquisition expenses	98,598	$ 76,761	$ 88,251	$ 83,282	74,146	$ 80,580	$ 69,005	$ 65,592	346,892	289,323	238,592
Net reinsurance costs and recoveries									(1,558,960)	(179,820)	(96,734)
Total gross reserve for claims and claim expenses	5,080,408				2,848,294				5,080,408	2,848,294	2,767,045
Distributions received									$ 29,700	$ 9,400	$ 13,300
Gross premiums written | Three Brokers
Related Party Transaction [Line Items]
Percent of gross premiums received									76.40%	80.80%	81.50%
Gross premiums written | AON
Related Party Transaction [Line Items]
Percent of gross premiums received									42.80%	46.40%	48.10%
Gross premiums written | Marsh
Related Party Transaction [Line Items]
Percent of gross premiums received									23.80%	23.60%	21.70%
Gross premiums written | Willis Towers Watson
Related Party Transaction [Line Items]
Percent of gross premiums received									9.80%	10.80%	11.70%
Tower Hill Companies
Related Party Transaction [Line Items]
Gross premiums written									$ 39,100	$ 32,800	$ 32,200
Gross premiums earned									35,700	32,300	35,800
Acquisition expenses									5,100	3,800	4,100
Related party receivables	14,100				14,200				14,100	14,200
Net reinsurance costs and recoveries									94,400	1,500	(1,600)
Total gross reserve for claims and claim expenses	$ 65,300				$ 36,800				65,300	36,800
Distributions received									8,300	9,000	13,100
Top Layer Reinsurance Ltd.
Related Party Transaction [Line Items]
Distributions received									20,000	0	0
Management fees revenue									$ 2,700	$ 2,600	$ 2,600