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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Financing Agreement Debt Instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Long-term debt	$ 200,000us-gaap_LongTermDebt	$ 280,500us-gaap_LongTermDebt	$ 100,000us-gaap_LongTermDebt
Total debt and capital lease obligations	226,148us-gaap_DebtAndCapitalLeaseObligations	287,623us-gaap_DebtAndCapitalLeaseObligations
Less: current maturities	2,528us-gaap_LongTermDebtCurrent	3,281us-gaap_LongTermDebtCurrent
Total long-term debt and capital lease obligations	223,620us-gaap_LongTermDebtNoncurrent	284,342us-gaap_LongTermDebtNoncurrent
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Long-term debt	0	80,500
Senior Notes \| Senior Unsecured Notes, 3.81 Percent, Due May 2018
Debt Instrument [Line Items]
Average interest rate	3.81%
Long-term debt	25,000	25,000
Senior Notes \| Senior Unsecured Notes, 4.60 Percent, Due May 2021
Debt Instrument [Line Items]
Average interest rate	4.60%
Long-term debt	75,000	75,000
Senior Notes \| Senior Unsecured Notes 3.13 Percent Due December 2020
Debt Instrument [Line Items]
Average interest rate	3.13%
Long-term debt	100,000	100,000
Capital lease obligations
Debt Instrument [Line Items]
Average interest rate	5.02%
Capital lease obligations	$ 26,148	$ 7,123