Debt (Details 4) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Series C convertible senior debentures and Senior Notes | Carrying Value
Notes and other indebtedness
Amount outstanding	$ 460.1
Series C convertible senior debentures and Senior Notes | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	507.7
Series C convertible senior debentures | Carrying Value
Notes and other indebtedness
Amount outstanding	90.1
Series C convertible senior debentures | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	106.7
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	41.1
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	112.3
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	70.4
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	$ 177.2