Debt (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Current portion of short-term debt	$ 1,200,000		$ 1,200,000
Unamortized discount	(1,220,000)		0
Total short-term debt	1,200,000		1,200,000
Long-term debt, excluding current maturities	1,669,380,000		1,342,100,000
Variable rate (2.276% at December 31, 2011) unsecured revolving credit facility, expires February 2016	0		671,500,000
Debt instrument, interest rate, stated percentage	7.15%
Total debt	1,670,580,000		1,343,300,000
Unsecured revolving credit facility, variable rate			2.276%
Purchase price upon exercise of option	101.00%
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 March 31, 2012.
Unsecured revolving credit facility, covenant terms	The Credit Facility requires additional subsidiary guarantors if certain guaranty coverage levels are not satisfied.  The revolving credit facility contains covenants which impose certain restrictions on the Company. 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. The terms of the Credit Facility also include covenants that restrict the ability of the Company and its material subsidiaries to merge, consolidate or sell all or substantially all of their assets, restrict the ability of the Company and its subsidiaries to incur liens and restrict the ability of the Company's subsidiaries to incur indebtedness.
Unsecured revolving credit facility, covenant compliance	At March 31, 2012, the Company's capital structure consisted of 29% debt and 71% equity and it was in compliance with the covenants of its debt agreements.  While the Company believes all of the lenders under the Credit Facility have the ability to provide funds, it cannot predict whether each will be able to meet its obligation under the facility.
Interest on debt	19,735,000	15,044,000
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 short-term debt	1,200,000		1,200,000
Total short-term debt	1,200,000		1,200,000
Long-term debt: Senior notes	30,600,000		30,600,000
Debt instrument, interest rate, stated percentage	7.15%
4.10% Senior Notes Due 2022 [Member]
Long-term debt: Senior notes	1,000,000,000		0
Debt instrument, interest rate, stated percentage	4.10%
Senior notes issued	$ 1,000,000,000