Debt (Details 3) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Senior Notes | Carrying Value
Notes and other indebtedness
Amount outstanding	$ 370.0	$ 370.0
Senior Notes | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	392.4	387.0
Senior Notes - Series A | Carrying Value
Notes and other indebtedness
Amount outstanding	40.0	40.0
Senior Notes - Series A | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	40.7	40.6
Senior Notes - Series B | Carrying Value
Notes and other indebtedness
Amount outstanding	105.0	105.0
Senior Notes - Series B | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	110.9	110.0
Senior Notes - Series C | Carrying Value
Notes and other indebtedness
Amount outstanding	65.0	65.0
Senior Notes - Series C | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	69.1	68.3
Senior Notes - Series D | Carrying Value
Notes and other indebtedness
Amount outstanding	160.0	160.0
Senior Notes - Series D | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	$ 171.7	$ 168.1