Fair Value of Debt (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	$ 1,174.3	$ 1,180.2
Secured Debt [Member] \| Carrying (Reported) Amount, Fair Value Disclosure [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	379.3	379.0
Secured Debt [Member] \| Estimate of Fair Value, Fair Value Disclosure [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	426.5	415.0
2017 Notes [Member] \| Carrying (Reported) Amount, Fair Value Disclosure [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	300.0	300.0
2017 Notes [Member] \| Estimate of Fair Value, Fair Value Disclosure [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	309.8	295.5
2019 Notes [Member] \| Carrying (Reported) Amount, Fair Value Disclosure [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	200.0	200.0
2019 Notes [Member] \| Estimate of Fair Value, Fair Value Disclosure [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	210.5	195.0
2014 Notes [Member] \| Carrying (Reported) Amount, Fair Value Disclosure [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	249.9	249.9
2014 Notes [Member] \| Estimate of Fair Value, Fair Value Disclosure [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	$ 256.9	$ 243.8

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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