Impact of Catastrophe Costs (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Impact of Catastrophe Costs
Net catastrophe costs	$ 66.4	$ 26.9	$ 611.8	$ 157.3
First Quarter 2011 Flooding in Australia and Cyclone Yasi
Impact of Catastrophe Costs
Net catastrophe costs			55.0
February 2011 Earthquake in New Zealand
Impact of Catastrophe Costs
Net catastrophe costs	61.0		182.0
March 2011 Tohoku earthquake and resulting tsunami in Japan
Impact of Catastrophe Costs
Net catastrophe costs	(23.0)		342.0
April 2010 Deepwater Horizon Explosion
Impact of Catastrophe Costs
Net catastrophe costs		14.0		14.0
February 2010 Earthquake in Chile
Impact of Catastrophe Costs
Net catastrophe costs				125.0
Severe second quarter 2011 tornado activity in the U.S.
Impact of Catastrophe Costs
Net catastrophe costs	$ 34.0