Variable Interest Entities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Total net assets attributable to XL	$ 108,735	$ 108,466
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Total net assets	225,220	242,605
Non-controlling interest in equity of consolidated subsidiaries	116,485	134,139
New Ocean Capital Management Ltd [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Non-controlling interest in equity of consolidated subsidiaries	$ 700	$ 500