Fair Value Measurement (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Total
Fair value
Financial liabilities	$ 885.4	$ 870.2
Total | Senior Secured Notes
Fair value
Financial liabilities	384.8	363.5
Total | Senior Unsecured Notes
Fair value
Financial liabilities	479.3	478.9
Level 1
Fair value
Financial liabilities	0	0
Level 1 | Senior Secured Notes
Fair value
Financial liabilities	0	0
Level 1 | Senior Unsecured Notes
Fair value
Financial liabilities	0	0
Level 2
Fair value
Financial liabilities	864.1	842.4
Level 2 | Senior Secured Notes
Fair value
Financial liabilities	384.8	363.5
Level 2 | Senior Unsecured Notes
Fair value
Financial liabilities	479.3	478.9
Level 3
Fair value
Financial liabilities	21.3	27.8
Level 3 | Senior Secured Notes
Fair value
Financial liabilities	0	0
Level 3 | Senior Unsecured Notes
Fair value
Financial liabilities	0	0
Tangible Equity Units | Total | tMEDS
Fair value
Financial liabilities	21.3	27.8
Tangible Equity Units | Level 1 | tMEDS
Fair value
Financial liabilities	0	0
Tangible Equity Units | Level 2 | tMEDS
Fair value
Financial liabilities	0	0
Tangible Equity Units | Level 3 | tMEDS
Fair value
Financial liabilities	$ 21.3	$ 27.8