Fair Value Measurements (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
ARO Trust investments	$ 41,700,000	$ 32,700,000
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	0	0
Long-term debt	0	0
Fair Value, Inputs, Level 2 [Member]
Additional Fair Value Elements [Abstract]
Notes receivable	5,000,000	6,300,000
Long-term debt	(1,601,700,000)	(1,512,900,000)
Fair Value, Inputs, Level 3 [Member]
Additional Fair Value Elements [Abstract]
Notes receivable	0	0
Long-term debt	0	0
Reported Value Measurement [Member]
Additional Fair Value Elements [Abstract]
Notes receivable	5,000,000	6,300,000
Long-term debt	(1,428,400,000)	(1,428,400,000)
Estimate of Fair Value Measurement [Member]
Additional Fair Value Elements [Abstract]
Notes receivable	5,000,000	6,300,000
Long-term debt	(1,601,700,000)	(1,512,900,000)
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	41,700,000	32,700,000
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	41,700,000	32,700,000
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	$ 41,700,000	$ 32,700,000