Debt (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Debt
Total senior notes	$ 847,673,000		$ 847,673,000		$ 875,173,000
Debt discount	(3,961,000)		(3,961,000)		(4,305,000)
Senior notes, net	843,712,000		843,712,000		870,868,000
Secured notes payable	8,789,000		8,789,000		9,010,000
Total debt	852,501,000		852,501,000		879,878,000
Redemption and repurchase of senior notes in cash	28,222,000		28,222,000	300,554,000
Gain (loss) on repurchase of debt	(857,000)	(19,071,000)	(857,000)	(19,308,000)
Letters of credit outstanding	71,100,000		71,100,000		74,300,000
Senior notes.
Debt
Face value of debt instrument redeemed		255,700,000
Redemption and repurchase of senior notes in cash		273,900,000
Charges resulting from the tender offer and redemption		19,500,000
Face value of additional debt instrument redeemed		19,000,000
Redemption and repurchase of additional senior notes in cash		18,400,000
Gain (loss) on repurchase of debt		433,000
6.9 percent senior notes due June 2013
Debt
Total senior notes	186,192,000		186,192,000		186,192,000
Interest rate stated percentage (as a percent)	6.90%		6.90%		6.90%
5.4 percent senior notes due January 2015
Debt
Total senior notes	131,481,000		131,481,000		158,981,000
Interest rate stated percentage (as a percent)	5.40%		5.40%		5.40%
Face value of debt instrument redeemed	27,500,000
Redemption and repurchase of senior notes in cash	28,222,000
Gain (loss) on repurchase of debt	(857,000)
8.4 percent senior notes due May 2017
Debt
Total senior notes	230,000,000		230,000,000		230,000,000
Interest rate stated percentage (as a percent)	8.40%		8.40%		8.40%
6.6 percent senior notes due May 2020
Debt
Total senior notes	$ 300,000,000		$ 300,000,000		$ 300,000,000
Interest rate stated percentage (as a percent)	6.60%		6.60%		6.60%