Debt (Details) (USD $)	1 Months Ended	6 Months Ended
	Feb. 28, 2011	Jun. 30, 2011	Dec. 31, 2010
Current portion of long-term debt: Senior notes		$ 1,200,000	$ 1,200,000
Total short-term debt		1,200,000	1,200,000
Long-term debt, excluding current maturities		1,215,400,000	1,093,000,000
Variable rate (2.165% at March 31, 2011 and 0.887% at December 31, 2011) unsecured revolving credit facility, expires February 2016		544,200,000	421,200,000
Total debt		1,216,600,000	1,094,200,000
Unsecured revolving credit facility, variable rate		2.165%	0.887%
Unsecured revolving credit facility, expiration date	February 2016
Unsecured revolving credit facility, borrowing capacity			1,500,000,000
Unsecured revolving credit facility, maximum borrowing capacity			2,000,000,000
Unsecured revolving credit facility, interest rate description		The interest rate on the amended credit facility is calculated based upon our debt rating and is currently 200 basis points over the current London Interbank Offered Rate (LIBOR) and was 200 basis points over LIBOR at June 30, 2011.
Unsecured revolving credit facility, covenant terms		Under the credit agreement, the Company may not issue total debt in excess of 60% of its total capital and must maintain a ratio of earnings before interest, taxes, depreciation and amortization (EBITDA) to interest expense of 3.5 or above.
Unsecured revolving credit facility, covenant compliance		At June 30, 2011, the Company's capital structure consisted of 27% debt and 73% equity and it was in compliance with the covenants of its debt agreements.
7.5% Senior Notes due 2018 [Member]
Long-term debt: Senior notes		600,000,000	600,000,000
Debt instrument, interest rate, stated percentage		7.50%
7.35% Senior Notes due 2017 [Member]
Long-term debt: Senior notes		15,000,000	15,000,000
Debt instrument, interest rate, stated percentage		7.35%
7.125% Senior Notes due 2017 [Member]
Long-term debt: Senior notes		25,000,000	25,000,000
Debt instrument, interest rate, stated percentage		7.125%
7.15% Senior Notes due 2018 [Member]
Current portion of long-term debt: Senior notes		1,200,000	1,200,000
Total short-term debt		1,200,000	1,200,000
Long-term debt: Senior notes		$ 31,200,000	$ 31,800,000
Debt instrument, interest rate, stated percentage		7.15%