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Quarterly cash dividends in excess of this amount results in an adjustment to the conversion rate on our 3.70% convertible debt.

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Increase for additional borrowings on the debt instrument during the period.

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Interest rate stated in the contractual debt agreement.

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Gross amount of debt extinguished.

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Amount represents the difference between the fair value of the payments made and the carrying amount of the debt at the time of its extinguishment.

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Unsecured convertible notes (Details 2) (USD $)	3 Months Ended		6 Months Ended			1 Months Ended					1 Months Ended	3 Months Ended		12 Months Ended
Unsecured convertible notes (Details 2) (USD $)	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jul. 31, 2010 8.00% Unsecured Convertible Notes	Jun. 30, 2010 8.00% Unsecured Convertible Notes	Apr. 30, 2009 8.00% Unsecured Convertible Notes	Jun. 30, 2011 8.00% Unsecured Convertible Notes	Dec. 31, 2010 8.00% Unsecured Convertible Notes	Jan. 31, 2007 3.70% Unsecured Convertible Notes.	Jun. 30, 2011 3.70% Unsecured Convertible Notes.	Mar. 31, 2011 3.70% Unsecured Convertible Notes.	Dec. 31, 2010 3.70% Unsecured Convertible Notes.	Apr. 30, 2009 3.70% Unsecured Convertible Notes.
Principal amount of unsecured convertible notes issued								$ 240,000,000			$ 460,000,000
Stated interest rate (as a percent)	8.00%	8.00%	8.00%	8.00%		8.00%	8.00%	8.00%	8.00%		3.70%	3.70%		3.70%	3.70%
Conversion price (in dollars per share)								$ 41.40	$ 41.40		$ 117.96	$ 117.36		$ 117.36
Denominator applied to principal							1,000	1,000	1,000	1,000	1,000	1,000		1,000
Exchange rate per $1000 principal amount							24.1546	24.1546			8.4774	8.5207
Cash premium (per $1,000 principal amount)							180
Principal amount of unsecured convertible notes retired							232,700,000
Percentage of aggregate principal amount retired (as a percent)							97.00%
Common stock shares issued pursuant to the terms of the Exchange Offer							5,620,256
Payment on exchange of unsecured convertible notes				43,528,000			41,900,000
Payment of accrued and unpaid interest on the retired portion of unsecured convertible notes							3,100,000
Principal amount of unsecured convertible notes repurchased						7,100,000							96,100,000	82,800,000
Gain (loss) on early extinguishment of debt	(1,248,000)	(41,496,000)	(3,743,000)	(41,496,000)		(1,300,000)	(41,500,000)						(2,500,000)	(2,400,000)
Unamortized issuance costs written off due to extinguishment of debt						140,000	4,700,000						400,000	400,000
Repurchase of unsecured convertible notes, aggregate cash price			98,590,000			12,800,000							98,600,000	84,600,000
Settlement consideration allocated to the reacquisition of the equity component						5,200,000							2,500,000	1,700,000
Minimum amount of cash dividend on common stock affecting conversion rate (in dollars per share)											$ 0.74
Net carrying amount of liability component	203,638,000		203,638,000		295,293,000				233,000	230,000		203,405,000		295,063,000
Unamortized discount									$ 17,000	$ 20,000		$ 2,472,000		$ 6,871,000