Segment Information (Gross Premiums Written By Geographic Location) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 863.5	$ 880.6
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	524.0	536.6
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	144.2	196.6
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	102.2	40.1
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	88.9	104.0
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	4.2	3.3
Switzerland
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 35.6	$ 40.4