Long-Term Debt - Outstanding Long-Term Debt (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Long-term Debt	$ 161,500us-gaap_LongTermDebt	$ 122,000us-gaap_LongTermDebt
Capital lease obligation	6,130us-gaap_CapitalLeaseObligations	6,978us-gaap_CapitalLeaseObligations
Long-term Debt and Capital Lease Obligations	167,595	128,945
Less: current maturities	(9,109)	(11,353)
Total long-term debt, net of current maturities	158,486us-gaap_LongTermDebtNoncurrent	117,592us-gaap_LongTermDebtNoncurrent
9.08% bond, due June 1, 2022
Debt Instrument [Line Items]
Long-term Debt	7,969	7,967
7.83% note, due January 1, 2015
Debt Instrument [Line Items]
Long-term Debt	0	2,000
6.64% note, due October 31, 2017
Debt Instrument [Line Items]
Long-term Debt	8,182	10,909
5.50% note, due October 12, 2020
Debt Instrument [Line Items]
Long-term Debt	12,000	14,000
5.93% note, due October 31, 2023
Debt Instrument [Line Items]
Long-term Debt	27,000	30,000
5.68% note, due June 30, 2026
Debt Instrument [Line Items]
Long-term Debt	29,000	29,000
6.43% note, due May 2, 2028
Debt Instrument [Line Items]
Long-term Debt	7,000	7,000
3.73% note, due December 16, 2028
Debt Instrument [Line Items]
Long-term Debt	20,000	20,000
3.88% note, due May 15, 2029
Debt Instrument [Line Items]
Long-term Debt	50,000	0
8.25% due March 1, 2014
Debt Instrument [Line Items]
Long-term Debt	0	646
Promissory note
Debt Instrument [Line Items]
Long-term Debt	$ 314	$ 445

Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of long-term debt and capital lease obligation due after one year or beyond the normal operating cycle, if longer.

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Amount of long-term debt and capital leases due within one year or the normal operating cycle, if longer.

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Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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