Organization Organization (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended		60 Months Ended
	Aug. 03, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2020	Apr. 14, 2015
Business Acquisition [Line Items]
Termination Fee Paid			$ 315.0
EXOR S.p.A. [Member]
Business Acquisition [Line Items]
BusinessAcquisitionPercentageOfVotingInterestsProposedToBeAcquired					100.00%
Business Acquisition, Share Price	$ 137.50				$ 130
Special Dividend Per Share	$ 3.00
EXOR S.p.A. [Member] | Subsequent Event [Member]
Business Acquisition [Line Items]
Preferred Stock Contingent Cash Payments		$ 42.7
Preferred Stock Contingent Cash Payments Per Share		$ 1.25
Maximum Earnings Distributable				67.00%
Axis Capital Holdings Ltd [Member]
Business Acquisition [Line Items]
Termination Fee Paid			$ 315.0