Segment Information (Gross Premiums Written By Geographic Location) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 860.9	$ 863.5
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	537.2	524.0
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	142.6	144.2
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	94.0	88.9
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	82.5	102.2
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	4.6	4.2
Switzerland
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 35.1	$ 35.6