Fair Value Measurements (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
ARO Trust investments	$ 32,700,000	$ 17,800,000
Fair Value, Transfers Between Level 1 and Level 2, Description and Policy [Abstract]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0	0
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	0	0
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount	0	0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount	0	0
Fair Value, Inputs, Level 1 [Member]
Additional Fair Value Elements [Abstract]
Notes Receivable, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Fair Value, Inputs, Level 2 [Member]
Additional Fair Value Elements [Abstract]
Notes Receivable, Fair Value Disclosure	6,300,000	8,200,000
Long-term Debt, Fair Value	(1,512,900,000)	(1,704,500,000)
Fair Value, Inputs, Level 3 [Member]
Additional Fair Value Elements [Abstract]
Notes Receivable, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Reported Value Measurement [Member]
Additional Fair Value Elements [Abstract]
Notes Receivable, Fair Value Disclosure	6,300,000	8,200,000
Long-term Debt, Fair Value	(1,428,400,000)	(1,428,300,000)
Estimate of Fair Value Measurement [Member]
Additional Fair Value Elements [Abstract]
Notes Receivable, Fair Value Disclosure	6,300,000	8,200,000
Long-term Debt, Fair Value	(1,512,900,000)	(1,704,500,000)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
ARO Trust investments	32,700,000	17,800,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
ARO Trust investments	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
ARO Trust investments	0	0
Fair Value, Measurements, Recurring [Member] | Reported Value Measurement [Member]
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
ARO Trust investments	32,700,000	17,800,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
ARO Trust investments	$ 32,700,000	$ 17,800,000