Debt Level 4 (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Bank credit facility due March 2016	$ 86,600	$ 86,600
Total debt	1,956,795	1,948,050
Less current maturities	(36,412)	(31,429)
Total long-term debt	1,920,383	1,916,621
Unsecured Debt [Member]
Debt Instrument [Line Items]
Unamortized pricing discounts and other	(3,977)	(4,292)
Unsecured Debt [Member] \| February 2013 [Member]
Debt Instrument [Line Items]
Senior Notes	0	4,757
Unsecured Debt [Member] \| April 2015 [Member]
Debt Instrument [Line Items]
Senior Notes	136,465	136,465
Unsecured Debt [Member] \| April 2016 [Member]
Debt Instrument [Line Items]
Senior Notes	197,377	197,377
Unsecured Debt [Member] \| June 2017 [Member]
Debt Instrument [Line Items]
Senior Notes	295,000	295,000
Unsecured Debt [Member] \| October 2018 [Member]
Debt Instrument [Line Items]
Senior Notes	250,000	250,000
Unsecured Debt [Member] \| May 2019 [Member]
Debt Instrument [Line Items]
Senior Notes	250,000	250,000
Unsecured Debt [Member] \| November 2020 [Member]
Debt Instrument [Line Items]
Senior Notes	200,000	200,000
Unsecured Debt [Member] \| November 2021 [Member]
Debt Instrument [Line Items]
Senior Notes	150,000	150,000
Unsecured Debt [Member] \| April 2027 [Member]
Debt Instrument [Line Items]
Senior Notes	200,000	200,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Bank credit facility due March 2016	86,600	86,600
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Obligations under capital leases	190,506	176,445
Mortgages [Member]
Debt Instrument [Line Items]
Mortgage notes and other debt, maturities through 2047	$ 4,824	$ 5,698

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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The amount of debt discount (net of debt premium) that was originally recognized at the issuance of the instrument that has yet to be amortized.

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Amount borrowed under the credit facility as of 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, 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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Including the current and noncurrent portions, the carrying value of notes payable which were initially due after one year or beyond the normal operating cycle, if longer, and which are not otherwise defined in the taxonomy.

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Including the current and noncurrent portions, 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). Senior note holders are paid off in full before any payments are made to junior note holders.

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