Financing Agreements Financing Arrangements, Interest Rates and Maturities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	4.82%
Capital Lease Obligations	$ 31,799	$ 7,123
Debt and Capital Lease Obligations	331,799	287,623
Capital Lease Obligations, Current	3,076	3,281
Long-term Debt and Capital Lease Obligations	328,723	284,342
Master Notes [Member] | Senior Unsecured Notes Three Point Eight One Percent Due May Twenty Eighteen [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.81%
Debt Instrument, Maturity Date	May 31, 2018
Long-term Debt	25,000	25,000
Master Notes [Member] | Senior Unsecured Notes Four Point Six Zero Percent Due May Twenty Twenty One [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.60%
Debt Instrument, Maturity Date	May 31, 2021
Long-term Debt	75,000	75,000
Master Notes [Member] | Senior Unsecured Notes Three Point One Three Percent Due December Twenty Twenty [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.13%
Debt Instrument, Maturity Date	Dec. 31, 2020
Long-term Debt	100,000	100,000
Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End	1.14%
Line of Credit Facility, Expiration Date	Jan. 31, 2018
Long-term Line of Credit	$ 100,000	$ 80,500
Minimum [Member]
Debt Instrument [Line Items]
Lease Expiration Date	Apr. 01, 2014
Maximum [Member]
Debt Instrument [Line Items]
Lease Expiration Date	Jul. 31, 2029