Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments Net Asset Value	$ 378.4	$ 537.2
Other investments - structured transactions	$ 142.0	$ 141.3
Spread on U.S. treasury for discounting deposit liabilities	0.469%	0.265%

Net asset value of investment in certain entities that calculate net asset value per share, including, but not limited to, by unit, membership interest, or other ownership interest. Investments include, but are not limited to, investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds. Excludes fair value disclosure.

No definition available.

Name:	xlgroup_InvestmentsNetAssetValue
Namespace Prefix:	xlgroup_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant