Segment Information (Gross Premiums Written By Geographic Location) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 730.2	$ 754.1	$ 2,394.1	$ 2,460.6
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	497.5	489.6	1,517.5	1,509.3
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	87.4	106.0	281.1	239.3
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	75.1	84.5	360.1	449.0
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	67.2	70.8	220.8	251.0
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	3.0	3.2	14.6	12.0
Switzerland
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 8.3	$ 12.2	$ 46.3	$ 56.9