Variable Interest Entities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Total net assets attributable to XL	$ 108,286	$ 108,466
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Total net assets	235,936	242,605
Non-controlling interest in equity of consolidated subsidiaries	127,650	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	$ 200	$ 500