Fair Value Measurement - Carrying and Fair Value Amounts (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012		Sep. 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	86,594	[2]	111,525	[2]	85,133	[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	226,110	[1]	243,118	[1]	243,219	[1]
Line of Credit Facility, Amount Outstanding	$ 70,166	[1]	$ 70,444	[1]	$ 0	[1]

[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.

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