Debt (Details 3) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Notes and other indebtedness
Amount outstanding	$ 1,038.4	$ 1,011.6
Series C convertible senior debentures and Senior Notes | Carrying Value
Notes and other indebtedness
Amount outstanding	461.9
Series C convertible senior debentures and Senior Notes | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	528.3
Series C convertible senior debentures
Notes and other indebtedness
Amount outstanding		90.1
Series C convertible senior debentures | Carrying Value
Notes and other indebtedness
Amount outstanding	91.9
Series C convertible senior debentures | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	123.6
Senior Notes - Series A | Carrying Value
Notes and other indebtedness
Amount outstanding	40.0
Senior Notes - Series A | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	41.3
Senior Notes - Series B | Carrying Value
Notes and other indebtedness
Amount outstanding	105.0
Senior Notes - Series B | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	113.3
Senior Notes - Series C | Carrying Value
Notes and other indebtedness
Amount outstanding	65.0
Senior Notes - Series C | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	71.1
Senior Notes - Series D | Carrying Value
Notes and other indebtedness
Amount outstanding	160.0
Senior Notes - Series D | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	$ 179.0