Segment Information (Gross Premiums Written By Geographic Location) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 800.3	$ 826.0	$ 1,663.8	$ 1,706.6
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	495.6	483.1	1,020.0	1,019.8
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	140.9	167.9	285.0	364.5
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	91.4	93.2	193.6	133.3
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	65.1	76.3	153.7	180.2
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	7.3	5.5	11.5	8.8
Switzerland
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 2.5	$ 4.3	$ 38.1	$ 44.7