Fair Value Measurement - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Retirement Obligation, Revision of Estimate	$ 2,810,000		$ (943,000)
Cost-method Investments, Other than Temporary Impairment	2,800,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Total	318,067,000	[1]	333,152,000	[1]
Liabilities, Fair Value Disclosure	0		0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Total	0	[2]	0	[2]
Liabilities, Fair Value Disclosure	0		0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Total	0	[3]	0	[3]
Liabilities, Fair Value Disclosure	313,562,000		250,541,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Total	318,067,000		333,152,000
Liabilities, Fair Value Disclosure	313,562,000		250,541,000
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	201,542,000		178,174,000
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	0		0
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	0		0
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	201,542,000		178,174,000
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	5,000,000		4,999,000
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	0		0
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	0		0
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Money market funds	5,000,000		4,999,000
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Held-to-maturity Securities, Fair Value Disclosure current and long-term	111,525,000		149,979,000
Notes Payable, Fair Value Disclosure	0		0
Line of Credit Facility, Amount Outstanding	0
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Held-to-maturity Securities, Fair Value Disclosure current and long-term	0		0
Notes Payable, Fair Value Disclosure	0		0
Line of Credit Facility, Amount Outstanding	0
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Held-to-maturity Securities, Fair Value Disclosure current and long-term	0		0
Notes Payable, Fair Value Disclosure	243,118,000		250,541,000
Line of Credit Facility, Amount Outstanding	70,444,000
Estimate of Fair Value, Fair Value Disclosure [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Held-to-maturity Securities, Fair Value Disclosure current and long-term	111,525,000		149,979,000
Notes Payable, Fair Value Disclosure	243,118,000		250,541,000
Line of Credit Facility, Amount Outstanding	$ 70,444,000

[1] Quoted prices in active markets for identical assets or liabilities.

[2] Observable inputs other than Level 1 prices such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

[3] Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.