Fair Value Measurements (Details) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2011
Dec. 31, 2010
Liability Class:  
Mutual fund investments percentage of fund invested in common stock of large-cap U.S (in hundredths)25.00%25.00%
Mutual fund investments percentage of fund invested in common stock of small-cap U.S. companies (in hundredths)10.00%15.00%
Mutual fund investments percentage of fund invested in common stock of international companies (in hundredths)5.00%5.00%
Mutual fund investments percentage of fund invested in debt securities (in hundredths)15.00%15.00%
Mutual fund investments percentage of fund invested in short-term investments (in hundredths)20.00%20.00%
Mutual fund investments percentage of fund invested in blended funds (in hundredths)25.00%20.00%
Fair Value, Measurements, Recurring [Member]
  
Asset Class:  
Mutual fund investments in a non-qualified savings plan$ 6.1[1]$ 6.2[2]
Derivatives - natural gas instruments00.3
Total Assets6.16.5
Liability Class:  
Liabilities related to non-qualified savings plan(6.1)(6.2)
Derivatives - natural gas instruments(4.9)(7.3)
Derivatives - interest rate swaps (0.6)
Total Liabilities(11.0)(14.1)
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
  
Asset Class:  
Mutual fund investments in a non-qualified savings plan6.1[1]6.2[2]
Derivatives - natural gas instruments00
Total Assets6.16.2
Liability Class:  
Liabilities related to non-qualified savings plan(6.1)(6.2)
Derivatives - natural gas instruments00
Derivatives - interest rate swaps 0
Total Liabilities(6.1)(6.2)
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
  
Asset Class:  
Mutual fund investments in a non-qualified savings plan0[1]0[2]
Derivatives - natural gas instruments00.3
Total Assets00.3
Liability Class:  
Liabilities related to non-qualified savings plan00
Derivatives - natural gas instruments(4.9)(7.3)
Derivatives - interest rate swaps (0.6)
Total Liabilities(4.9)(7.9)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
  
Asset Class:  
Mutual fund investments in a non-qualified savings plan0[1]0[2]
Derivatives - natural gas instruments00
Total Assets00
Liability Class:  
Liabilities related to non-qualified savings plan00
Derivatives - natural gas instruments00
Derivatives - interest rate swaps 0
Total Liabilities00
Senior Notes [Member]
  
Fair Value of Outstanding Debt [Line Items]  
Senior Notes, interest rate8.00% 
Fair value of Senior Notes106.0 
Aggregate principal amount due at maturity - outstanding debt100.0 
Credit Agreement [Member]
  
Fair Value of Outstanding Debt [Line Items]  
Fair value of Senior Notes370.8 
Aggregate principal amount due at maturity - outstanding debt$ 385.8 
[1]Includes mutual fund investments of approximately 25% in the common stock of large-cap U.S. companies, approximately 10% in the common stock of small-cap U.S. companies, approximately 5% in the common stock of international companies, approximately 15% in debt securities of U.S. companies, approximately 20% in short-term investments and approximately 25% in blended funds.
[2]Includes mutual fund investments of approximately 25% in the common stock of large-cap U.S. companies, approximately 15% in the common stock of small-cap U.S. companies, approximately 5% in the common stock of international companies, approximately 15% in debt securities of U.S. companies, approximately 20% in short-term investments and approximately 20% in blended funds.