Credit Arrangements (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Credit arrangements
Total long-term debt	$ 242,938us-gaap_LongTermDebt	$ 219,526us-gaap_LongTermDebt
Less: current portion	(38,909)us-gaap_LongTermDebtCurrent	(28,475)us-gaap_LongTermDebtCurrent
Long-term Debt, Excluding Current Maturities, Total	204,029us-gaap_LongTermDebtNoncurrent	191,051us-gaap_LongTermDebtNoncurrent
Scheduled maturities of long-term debt
2015	38,909
2016	46,315
2017	41,282
2018	38,566
2019	33,965
Thereafter	43,901
Commercial equipment notes payable, maturing range from November 30, 2016 to December 13, 2020
Credit arrangements
Total long-term debt	112,420	144,526
Monthly payment of principal and interest	2,521	2,521
Commercial equipment notes payable, maturing range from November 30, 2016 to December 13, 2020 \| Minimum
Credit arrangements
Debt Instrument, Interest Rate, Stated Percentage	1.78%	1.78%
Commercial equipment notes payable, maturing range from November 30, 2016 to December 13, 2020 \| Maximum
Credit arrangements
Debt Instrument, Interest Rate, Stated Percentage	3.51%	3.51%
Commercial equipment notes payable, maturing range from March 31, 2019 to September 24, 2021
Credit arrangements
Total long-term debt	55,518
Monthly payment of principal and interest	999
Commercial equipment notes payable, maturing range from March 31, 2019 to September 24, 2021 \| Minimum
Credit arrangements
Debt Instrument, Interest Rate, Stated Percentage	1.94%
Commercial equipment notes payable, maturing range from March 31, 2019 to September 24, 2021 \| Maximum
Credit arrangements
Debt Instrument, Interest Rate, Stated Percentage	2.75%
Senior secured notes, maturing on December 28, 2022
Credit arrangements
Total long-term debt	50,000	50,000
Debt Instrument, Interest Rate, Stated Percentage	3.65%	3.65%
Senior secured notes, maturing on July 25, 2023
Credit arrangements
Total long-term debt	$ 25,000	$ 25,000
Debt Instrument, Interest Rate, Stated Percentage	3.85%

Contractual interest rate for funds borrowed, under the debt agreement.

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