Fair Value Measurement - Carrying and Fair Value Amounts (Details) - USD ($) $ in Thousands		Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Level 1 | Other Assets, Liabilites, Fair Value
Balance Sheet Grouping
Held-to-maturity marketable securities		$ 107,770	$ 105,336	$ 88,313
Level 1 | Other Assets Liabilities, Carrying Value
Balance Sheet Grouping
Held-to-maturity marketable securities		107,771	105,695	88,253
Level 3 | Other Assets, Liabilites, Fair Value
Balance Sheet Grouping
Senior notes payable	[1]	169,094	165,731	212,919
Credit Agreement loan	[1]	96,151	99,375	69,753
Level 3 | Other Assets Liabilities, Carrying Value
Balance Sheet Grouping
Senior notes payable		160,000	160,000	200,000
Credit Agreement loan		$ 96,250	$ 100,000	$ 70,000

[1] The fair values of the 2019 Notes and Credit Agreement (defined in Note 10) are based on borrowing rates available to us for long-term debt with similar terms, average maturities, and credit risk.