Debt (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
7.15 % Senior Notes due 2018	$ 1,200,000	$ 1,200,000
Total short-term debt	1,200,000	1,200,000
Variable rate (2.276% at December 31, 2011 and 0.887% at December 31, 2010) unsecured revolving credit facility, expires February 2016	671,500,000	421,200,000
Long-term debt, excluding current maturities	1,342,100,000	1,093,000,000
Total debt	1,343,300,000	1,094,200,000
Unsecured revolving credit facility, variable rate	2.276%	0.887%
2012	1,200,000
2013	1,200,000
2014	1,200,000
2015	1,200,000
2016	672,700,000
Thereafter	665,800,000
Purchase price upon exercise of option				101.00%
Senior notes maturity date				Feb. 01, 2018
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 December 31, 2011.
Unsecured revolving credit facility, covenant terms	The Credit Facility is guaranteed by the Company's subsidiary, SEECO. 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 December 31, 2011, the Company's capital structure consisted of 25% debt and 75% 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
Amount outstanding under revolving credit facility	671,500,000	421,200,000
Interest on debt	65,421,000	57,144,000	55,581,000
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%
Senior notes issued				600,000,000
7.35% Senior Notes due 2017 [Member]
Long-term debt: Senior notes	15,000,000	15,000,000
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]
7.15 % Senior Notes due 2018	1,200,000	1,200,000
Total short-term debt	1,200,000	1,200,000
Long-term debt: Senior notes	$ 30,600,000	$ 31,800,000