Fair Value Measurement - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013		Jun. 30, 2013		Dec. 31, 2012		Jun. 30, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Impairment Charges	$ 0		$ 0
Liabilities, Fair Value Disclosure [Abstract]
Line of Credit Facility, Amount Outstanding	70,000,000	[1]	70,000,000	[1]	70,000,000	[1]	0	[1]
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure	144,605,000		144,605,000		206,542,000		172,424,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure	0		0		0		0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure	0		0		0		0
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure	144,605,000		144,605,000		206,542,000		172,424,000
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Inputs, Level 3 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Senior Notes Payable, Fair Value Disclosure	227,902,000	[1]	227,902,000	[1]	243,118,000	[1]	239,443,000	[1]
Line of Credit Facility, Amount Outstanding	69,321,000	[1]	69,321,000	[1]	70,444,000	[1]	0	[1]
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	144,605,000		144,605,000		201,542,000		167,427,000
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0		0		0		0
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0		0		0		0
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	144,605,000		144,605,000		201,542,000		167,427,000
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents					5,000,000		4,997,000
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents					0		0
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents					0		0
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents					$ 5,000,000		$ 4,997,000

[1] 2The fair values of the senior notes payable and Credit Agreement loan are based on borrowing rates available to us for long-term loans with similar terms, average maturities, and credit risk.