Related Party Transactions - Schedule of related party transactions activity (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) management_company	Mar. 31, 2017 USD ($)		Dec. 31, 2017 management_company
Related Party Transaction [Line Items]
Number of investment management companies | management_company	2			4
Number of investment technology service firms | management_company	1			1
Reported net premiums written	$ 3,155,718	$ 2,983,070
Net losses and loss expenses incurred	1,622,006	1,583,456
Reported net acquisition costs	$ 463,827	435,869	[1]
XL Innovate Fund [Member]
Related Party Transaction [Line Items]
Ownership percentage held by noncontrolling interest	5.21%
Investment manager affiliates
Related Party Transaction [Line Items]
Reported net premiums written	$ 15,578	21,715
Net losses and loss expenses incurred	11,719	11,250
Reported net acquisition costs	$ 5,587	$ 9,214

[1] The Company has reflected the amortization of certain fair value adjustments recorded in conjunction with the Company's acquisition of Catlin Group Ltd in 2015 ("Catlin Acquisition") within the respective segments.