Fair Value Measurement - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Liabilities, Fair Value Disclosure [Abstract]
Line of Credit Facility, Amount Outstanding		$ 70,000	$ 70,000	$ 70,000	$ 70,000	$ 70,000
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Held-to-maturity Securities, Fair Value Disclosure current and long-term		88,075	111,999	88,075	111,999	101,808
Fair Value, Inputs, Level 3 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Senior Notes Payable, Fair Value Disclosure	[1]	212,648	222,920	212,648	222,920	220,226
Line of Credit Facility, Amount Outstanding	[1]	69,387	69,781	69,387	69,781	70,153
Fair Value, Measurements, Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure		23,975	31,583	23,975	31,583	60,618
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure		23,975	31,583	23,975	31,583	60,618
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Assets, Fair Value Disclosure		0	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	0
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents		23,975	31,583	23,975	31,583	60,618
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash equivalents		23,975	60,618	23,975	60,618	31,583
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	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	$ 0
Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, Gain (Loss) on Derivative, Net		$ 100	$ (1,200)	$ (1,300)	$ (900)

[1] 1The fair values of the senior notes payable and Credit Agreement (defined in Note 11) loan are based on borrowing rates available to us for long-term loans with similar terms, average maturities, and credit risk.