Related Party Transactions - Schedule of related party transactions activity (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) management_company	Sep. 30, 2016 USD ($) management_company	Sep. 30, 2017 USD ($) management_company	Sep. 30, 2016 USD ($) 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	$ 2,329,367	$ 2,135,658	$ 7,961,702	$ 7,926,536
Net losses and loss expenses incurred	3,066,723	1,491,803	6,179,262	4,506,674
Reported net acquisition costs	$ 445,685	403,888	$ 1,315,138	1,227,675
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	$ 20,233	14,491	$ 85,990	60,397
Net losses and loss expenses incurred	10,750	8,927	42,558	24,823
Reported net acquisition costs	$ 6,745	$ 6,115	$ 34,699	$ 25,550