Fair Value Measurement - Carrying and Fair Value Amounts (Details) (USD $)																											3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Sep. 30, 2014 Notes Payable to Banks [Member] Institutional Group Two [Member]	Sep. 30, 2014 London Interbank Offered Rate (LIBOR) [Member] Notes Payable to Banks [Member] Institutional Group Two [Member]	Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Money Market Funds [Member] Fair Value, Inputs, Level 1 [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Money Market Funds [Member] Fair Value, Inputs, Level 1 [Member]	Sep. 30, 2013 Fair Value, Measurements, Recurring [Member] Money Market Funds [Member] Fair Value, Inputs, Level 1 [Member]	Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Money Market Funds [Member] Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Money Market Funds [Member] Fair Value, Inputs, Level 3 [Member]	Sep. 30, 2013 Fair Value, Measurements, Recurring [Member] Money Market Funds [Member] Fair Value, Inputs, Level 3 [Member]	Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Money Market Funds [Member] Estimate of Fair Value, Fair Value Disclosure [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Money Market Funds [Member] Estimate of Fair Value, Fair Value Disclosure [Member]	Sep. 30, 2013 Fair Value, Measurements, Recurring [Member] Money Market Funds [Member] Estimate of Fair Value, Fair Value Disclosure [Member]	Sep. 30, 2014 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 1 [Member]		Dec. 31, 2013 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 1 [Member]		Sep. 30, 2013 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 1 [Member]		Sep. 30, 2014 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 3 [Member]		Dec. 31, 2013 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 3 [Member]		Sep. 30, 2013 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Inputs, Level 3 [Member]		Sep. 30, 2014 Swap [Member]	Sep. 30, 2014 Swap [Member]	Sep. 30, 2014 Commodity Contract [Member]	Sep. 30, 2014 Commodity Contract [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents						$ 50,148,000	$ 89,336,000	$ 113,220,000	$ 0	$ 0	$ 0	$ 50,148,000	$ 89,336,000	$ 113,220,000
Derivative, Notional Amount	100,000,000
Held-to-maturity Securities, Fair Value Disclosure current and long-term															101,711,000	[1]	116,915,000	[1]	86,594,000	[1]
Senior Notes	200,000,000	200,000,000	200,000,000
Notes Payable, Fair Value Disclosure																					225,186,000	[2]	225,865,000	[2]	226,110,000	[2]
Line of Credit Facility, Amount Outstanding	70,000,000	70,000,000	70,000,000																		70,258,000	[2]	69,601,000	[2]	70,166,000	[2]
Debt Instrument, Interest Rate, Stated Percentage				6.11%
Derivative, Variable Interest Rate					4.15%
Net gain (loss) on interest rate swap																											$ (600,000)	$ 300,000	$ 600,000	$ 700,000

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

[2] 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.