Long-Term Debt (Narrative) (Details) (USD $)	6 Months Ended	3 Months Ended		6 Months Ended	3 Months Ended	6 Months Ended		3 Months Ended
	Jun. 30, 2011	Jun. 30, 2011 Cash [Member] If Converted [Member] 2.875% Convertible Senior Notes [Member]	Jun. 30, 2011 Cash [Member] 2.875% Convertible Senior Notes [Member]	Jun. 30, 2011 Noncash [Member] If Converted [Member] 2.875% Convertible Senior Notes [Member]	Jun. 30, 2011 If Converted [Member] 2.875% Convertible Senior Notes [Member]	Jun. 30, 2011 Total Debt To Book Capitalization [Member]	Jun. 30, 2011 Company's Oil And Gas Properties To Total Debt [Member]	Jun. 30, 2011 2.875% Convertible Senior Notes [Member]	Dec. 31, 2010 2.875% Convertible Senior Notes [Member]
Line of credit facility, maximum borrowing capacity	$ 1,250,000,000
Outstanding borrowings under the Credit Facility	0
Undrawn letters of credit	65,100,000
Unused borrowing capacity	1,200,000,000
Aggregate outstanding swing line loans maximum	150,000,000
Credit facility, description	(a) a rate per annum equal to the higher of the prime rate announced from time to time by Wells Fargo Bank, National Association or the weighted average of the rates on overnight Federal funds transactions with members of the Federal Reserve System during the last preceding business day plus 0.5 percent plus a defined alternate base rate spread margin ("ABR Margin"), which is currently one percent based on the Company's debt rating or (b) a base Eurodollar rate, substantially equal to LIBOR, plus a margin (the "Applicable Margin"), which is currently two percent and is also determined by the Company's debt rating. Swing line loans under the Credit Facility bear interest at a rate per annum equal to the "ASK" rate for Federal funds periodically published by the Dow Jones Market Service plus the Applicable Margin. Letters of credit outstanding under the Credit Facility are subject to a per annum fee, representing the Applicable Margin plus 0.125 percent. The Company also pays commitment fees on undrawn amounts under the Credit Facility that are determined by the Company's debt rating (currently 0.375 percent).
Loss on extinguishment of debt to write off the unamortized issuance costs	2,400,000
Debt instrument covenant description						.60 to 1.0	1.75 to 1.0
Senior notes, interest rate								2.875%	2.875%
Senior convertible notes, outstanding								480,000,000	480,000,000
Senior convertible notes, principal amount								70,000
Shares received for conversion of debt					1,800,000			340
Days included in calculation	20
Value of shares received for conversion of debt		$ 479,900,000	$ 71,000	$ 158,800,000