Acquisitions (Other Details) (Details) - USD ($)		1 Months Ended	9 Months Ended	12 Months Ended
	Apr. 02, 2015	Jan. 31, 2015	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Apr. 30, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Goodwill			$ 388,100,000	$ 389,700,000	$ 388,100,000			$ 278,300,000
Finite-lived Intangible Assets Acquired				$ 0	$ 83,500,000
Hong Kong And Singapore Operations Of Royal Sun Alliance Insurance Plc
Business Acquisition [Line Items]
Cash consideration	$ 176,500,000
Cash Acquired from Acquisition			17,400,000
Business Combination, Acquisition Related Costs			$ 9,200,000
Goodwill	106,200,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life				12 years
Insurance Related Intangible Liability				$ 8,300,000
Insurance Related Intangible Liability Amortization Period				8 years
Latin American Underwriters Holdings Ltd [Member]
Business Acquisition [Line Items]
Cash consideration		$ 5,100,000
Goodwill		2,500,000
Finite-lived Intangible Assets Acquired		$ 3,600,000
Estimated useful life		3 years
Business Combination, Contingent Consideration, Liability		$ 1,000,000				$ 0
HONG KONG | Hong Kong And Singapore Operations Of Royal Sun Alliance Insurance Plc
Business Acquisition [Line Items]
Goodwill	54,700,000
SINGAPORE | Hong Kong And Singapore Operations Of Royal Sun Alliance Insurance Plc
Business Acquisition [Line Items]
Goodwill	$ 51,500,000
LABUAN | Hong Kong And Singapore Operations Of Royal Sun Alliance Insurance Plc
Business Acquisition [Line Items]
Goodwill							$ 1,400,000