Related Party Transactions - Schedule of related party transactions activity (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) management_company	Jun. 30, 2017 USD ($) management_company	Jun. 30, 2018 USD ($) management_company	Jun. 30, 2017 USD ($) management_company
Related Party Transaction [Line Items]
Document Fiscal Year Focus			2018
Number of investment management companies | management_company	2	3	2	3
Number of investment technology service firms | management_company	1	1	1	1
Reported net premiums written	$ 2,821,744	$ 2,649,265	$ 5,977,462	$ 5,632,335
Net losses and loss expenses incurred	1,721,309	1,529,083	3,343,315	3,112,539
Reported net acquisition costs	$ 471,491	433,584	$ 935,318	869,453	[1]
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	$ 50,024	44,042	$ 65,602	65,757
Net losses and loss expenses incurred	27,570	20,558	39,289	31,808
Reported net acquisition costs	$ 21,946	$ 18,741	$ 27,534	$ 27,955

[1] The Company has reflected the amortization of certain fair value adjustments recorded in conjunction with the Catlin Acquisition within the respective segments