Fair Value Measurement - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013		Sep. 30, 2013		Dec. 31, 2012		Sep. 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	113,220,000		113,220,000		206,542,000		200,081,000
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	113,220,000		113,220,000		206,542,000		200,081,000
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	226,110,000	[1]	226,110,000	[1]	243,118,000	[1]	243,219,000	[1]
Line of Credit Facility, Amount Outstanding	70,166,000	[1]	70,166,000	[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	113,220,000		113,220,000		201,542,000		185,586,000
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	113,220,000		113,220,000		201,542,000		185,586,000
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents					5,000,000		14,495,000
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		$ 14,495,000

[1] 2The fair values of the senior notes payable and Credit Agreement (as defined under “Credit Agreement” in “Item 2. Management’s Discussion and Analysis of Financial Condition and Results of Operations”) loan are based on borrowing rates available to us for long-term loans with similar terms, average maturities, and credit risk.