Fair Value of Financial Instruments (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, less current maturities	$ 1,202,063,000	$ 1,411,448,000
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, less current maturities at fair value	$ 1,334,200,000	$ 1,486,200,000