Fair Value Measurement - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013		Mar. 31, 2013
Liabilities, Fair Value Disclosure [Abstract]
Line of Credit Facility, Amount Outstanding	$ 70,000		$ 70,000		$ 70,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure	49,770		89,336		165,844
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure	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
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure	49,770		89,336		165,844
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	226,503	[1]	225,865	[1]	242,071	[1]
Line of Credit Facility, Amount Outstanding	69,368	[1]	69,601	[1]	70,568
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	49,770		89,336		160,845
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
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
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	49,770		89,336		160,845
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents					4,999
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents					0
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents					0
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents					$ 4,999

[1] 2The fair values of the senior notes payable and Credit Agreement (defined in Note 10) loan are based on borrowing rates available to us for long-term loans with similar terms, average maturities, and credit risk.