Debt Level 4 (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Bank credit facility due March 2016	$ 86,600	$ 86,600
Total debt	1,950,697	1,948,050
Less current maturities	(37,315)	(31,429)
Total long-term debt	1,913,382	1,916,621
Unsecured Debt [Member]
Debt Instrument [Line Items]
Unamortized pricing discounts and other	(4,135)	(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
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Obligations under capital leases	183,883	176,445
Mortgages [Member]
Debt Instrument [Line Items]
Mortgage notes and other debt, maturities through 2047	$ 5,507	$ 5,698