Related Party Transactions - Schedule of related party transactions activity (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) management_company	Mar. 31, 2016 USD ($)	Dec. 31, 2016 management_company
Related Party Transaction [Line Items]
Number of investment management companies | management_company	4		6
Number of investment technology service firms | management_company	1		2
Reported net premiums written	$ 2,983,070	$ 3,064,759
Net losses and loss expenses incurred	1,583,456	1,382,485
Reported net acquisition costs	$ 435,869	403,267
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	$ 21,715	24,016
Net losses and loss expenses incurred	11,250	12,060
Reported net acquisition costs	$ 9,214	$ 11,604