Related Party Transactions - Schedule of related party transactions activity (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) management_company	Jun. 30, 2016 USD ($) management_company	Jun. 30, 2017 USD ($) management_company	Jun. 30, 2016 USD ($) management_company
Related Party Transaction [Line Items]
Number of investment management companies | management_company	3	4	3	4
Number of investment technology service firms | management_company	1	1	1	1
Reported net premiums written	$ 2,649,265	$ 2,726,119	$ 5,632,335	$ 5,790,878
Net losses and loss expenses incurred	1,529,083	1,632,386	3,112,539	3,014,871
Reported net acquisition costs	$ 433,584	420,520	$ 869,453	823,787
XL Innovate Fund [Member]
Related Party Transaction [Line Items]
Ownership percentage held by noncontrolling interest	5.21%		5.21%
Investment manager affiliates
Related Party Transaction [Line Items]
Reported net premiums written	$ 44,042	21,889	$ 65,757	45,905
Net losses and loss expenses incurred	20,558	3,836	31,808	15,896
Reported net acquisition costs	$ 18,741	$ 7,831	$ 27,955	$ 19,435