Fair Value Measurement - Carrying and Fair Value Amounts (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012		Jun. 30, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	$ 200,000	[1]	$ 208,333	[1]	$ 208,333	[1]
Line of Credit Facility, Amount Outstanding	70,000	[1]	70,000	[1]	0	[1]
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity Securities, Fair Value Disclosure current and long-term	76,100	[2]	111,525	[2]	89,239	[2]
Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Payable, Fair Value Disclosure	227,902	[1]	243,118	[1]	239,443	[1]
Line of Credit Facility, Amount Outstanding	$ 69,321	[1]	$ 70,444	[1]	$ 0	[1]

[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.

[2] 1Held-to-maturity marketable securities are periodically assessed for other-than-temporary impairment.