Financing Arrangements, Interest Rates and Maturities (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Effective interest rate	5.04%
Capital lease obligations	$ 22,877,000us-gaap_CapitalLeaseObligations	$ 26,148,000us-gaap_CapitalLeaseObligations
Total debt, capital lease obligations, and notes	329,065,000us-gaap_DebtAndCapitalLeaseObligations	226,148,000us-gaap_DebtAndCapitalLeaseObligations
Less: current maturities	2,203,000us-gaap_CapitalLeaseObligationsCurrent	2,528,000us-gaap_CapitalLeaseObligationsCurrent
Total long-term debt, capital lease obligations, and notes payable	326,862,000	223,620,000
Senior Notes \| Senior Unsecured Notes 3.81% Due May 2018
Debt Instrument [Line Items]
Stated interest rate	3.81%
Long-term Debt	25,000,000	25,000,000
Senior Notes \| Senior Unsecured Notes 4.60% Due May 2021
Debt Instrument [Line Items]
Stated interest rate	4.60%
Long-term Debt	75,000,000	75,000,000
Senior Notes \| Senior Unsecured Notes 3.13% Due December 2020
Debt Instrument [Line Items]
Stated interest rate	3.13%
Long-term Debt	100,000,000	100,000,000
Notes payable
Debt Instrument [Line Items]
Stated interest rate	3.50%
Long-term Debt	14,500,000	0
Revolving Credit Facility
Debt Instrument [Line Items]
Average interest rate at March 31, 2015	0.90%
Long-term Line of Credit	$ 91,688,000	$ 0

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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Amount of capital lease obligation due within one year or the normal operating cycle, if longer.

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Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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

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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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The effective interest rate at the end of the reporting period.

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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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