Related Party Transactions and Major Customers (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) customer	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Related Party Transaction [Line Items]
Gross premiums written	$ 323,091	$ 430,224	$ 759,128	$ 862,133	$ 336,093	$ 369,642	$ 661,997	$ 643,578	$ 2,374,576	$ 2,011,310	$ 1,550,572
Gross premiums earned									1,874,993	1,769,088	1,450,047
Acquisition expenses	74,146	$ 80,580	$ 69,005	$ 65,592	55,399	$ 78,126	$ 61,666	$ 43,401	289,323	238,592	144,476
Net reinsurance costs and recoveries									(179,820)	(96,734)	(30,634)
Reserve for claims and claim expenses	2,848,294				2,767,045				2,848,294	2,767,045	1,412,510
Distributions received									$ 9,400	$ 13,300	$ 10,300
Gross premiums written
Related Party Transaction [Line Items]
Number of significant customers | customer									3
Percent of gross premiums received									80.80%	81.50%	87.20%
Gross premiums written | AON
Related Party Transaction [Line Items]
Percent of gross premiums received									46.40%	48.10%	51.50%
Gross premiums written | Marsh
Related Party Transaction [Line Items]
Percent of gross premiums received									23.60%	21.70%	21.50%
Gross premiums written | Willis Towers Watson
Related Party Transaction [Line Items]
Percent of gross premiums received									10.80%	11.70%	14.20%
Tower Hill Companies
Related Party Transaction [Line Items]
Gross premiums written									$ 32,800	$ 32,200	$ 40,000
Gross premiums earned									32,300	35,800	41,900
Acquisition expenses									3,800	4,100	4,700
Related party receivables	14,200				14,300				14,200	14,300
Net reinsurance costs and recoveries									(1,500)	1,600	3,600
Reserve for claims and claim expenses	$ 36,800				$ 38,200				36,800	38,200
Distributions received									9,000	13,100
Top Layer Reinsurance Ltd.
Related Party Transaction [Line Items]
Distributions received									0	0	0
Management fees revenue									$ 2,600	$ 2,600	$ 2,800