Fair Value Measurement - Carrying and Fair Value Amounts (Details) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Reported Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity marketable securities		$ 88,253	$ 102,067
Reported Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes payable	[1]	200,000	200,000	$ 200,000
Credit Agreement loan	[1]	70,000	70,000	70,000
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity marketable securities		88,313		101,711
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes payable	[1]	212,919	220,226	225,186
Credit Agreement loan	[1]	$ 69,753	$ 70,153	$ 70,258

[1] The 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.