Long-Term Debt Schedule of Debt (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
Jun. 30, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Senior revolving credit facility maturing in 2019	$ 135,000	$ 110,000
Total	288,334	267,334
Amounts due within one year	6,667	6,667
Long-term debt	281,667	260,667
Debt instruments maturity date	2016	2016
Industrial Development Debt Payable Through 2022, 0.30%
Debt Instrument [Line Items]
Industrial development debt issues payable through 2022, 0.30%	0	4,000
Debt instruments maturity date	2022	2022
Debt instrument, stated interest rate percentage	0.30%	0.30%
Senior Notes Payable Through 2021, 5.41%
Debt Instrument [Line Items]
Senior notes payable	53,334	53,334
Debt instruments maturity date	2021	2021
Debt instrument, stated interest rate percentage	5.41%	5.41%
Senior Notes Payable Through 2021, 4.00%
Debt Instrument [Line Items]
Senior notes payable	$ 100,000	$ 100,000
Debt instruments maturity date	2021	2021
Debt instrument, stated interest rate percentage	4.00%	4.00%

Debt instruments maturity date.

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- Definition

Industrial development debt issues payable.

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- Definition

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

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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- Definition

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, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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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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Carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

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