Significant Accounting Policies (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) segments country $ / shares	Dec. 31, 2015 USD ($) $ / shares	Dec. 31, 2014 USD ($)
Significant Accounting Policies [Line Items]
Number of Reporting Units | segments	2
Number of Countries in which Entity Operates | country	29
Premiums Earned and Associated Tax Accruals Adjustment Amount | $			$ 20,000
Options, life of award	10 years
Number of Operating Segments | segments	2
Common shares, par value (in dollars per share)	$ 0.01	$ 0.01
Hedge funds
Significant Accounting Policies [Line Items]
Alternative investments, timelag for reporting	1 month
Private investment funds
Significant Accounting Policies [Line Items]
Alternative investments, timelag for reporting	3 months
Operating affiliates
Significant Accounting Policies [Line Items]
Alternative investments, timelag for reporting	3 months
Maximum
Significant Accounting Policies [Line Items]
Vesting period required	4 years
Minimum
Significant Accounting Policies [Line Items]
Vesting period required	3 years
XL Group Ltd
Significant Accounting Policies [Line Items]
Common shares, par value (in dollars per share)	$ 0.01
XL Ireland [Member]
Significant Accounting Policies [Line Items]
Common shares, par value (in dollars per share)	$ 0.01
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains (Losses) on Investments
Significant Accounting Policies [Line Items]
Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Investments Transferred From Available for sale To Equity Method | $	$ 33,530	$ 0