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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Significant Accounting Policies (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 segments country $ / shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($)	May 30, 2014
Significant Accounting Policies [Line Items]
Number of countries in which entity operates \| country		29
Number of reporting units \| segments		2
Number of operating segments \| segments		2
Common shares, par value (in dollars per share) \| $ / shares		$ 0.01	$ 0.01
Options, life of award		10 years
Prior period reclassification adjustment \| $			$ (28.6)	$ (28.9)
Hedge funds
Significant Accounting Policies [Line Items]
Alternative investments, timelag for reporting		1 month
Private investments
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) \| $ / shares		$ 0.01
XL Ireland [Member]
Significant Accounting Policies [Line Items]
Common shares, par value (in dollars per share) \| $ / shares		$ 0.01
GreyCastle Life Reinsurance SAC Ltd
Significant Accounting Policies [Line Items]
Quota share reinsurance percentage					100.00%
Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Cumulative effect on retained earnings, before tax \| $	$ 235.2
Cumulative effect on retained earnings, tax \| $	$ 12.1