Unsecured convertible notes (Details 2) (USD $)	3 Months Ended		9 Months Ended			1 Months Ended						1 Months Ended	3 Months Ended		12 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Jul. 31, 2010 8.00% Unsecured Convertible Notes	Jun. 30, 2010 8.00% Unsecured Convertible Notes rate	Apr. 30, 2009 8.00% Unsecured Convertible Notes rate	Sep. 30, 2011 8.00% Unsecured Convertible Notes	Jul. 31, 2011 8.00% Unsecured Convertible Notes	Dec. 31, 2010 8.00% Unsecured Convertible Notes	Jan. 31, 2007 3.70% Unsecured Convertible Notes. rate	Sep. 30, 2011 3.70% Unsecured Convertible Notes. rate	Mar. 31, 2011 3.70% Unsecured Convertible Notes.	Dec. 31, 2010 3.70% Unsecured Convertible Notes.	Sep. 30, 2010 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				$ 117.36		$ 117.36	$ 117.36
Numerator applied to principal								24.1887				8.5207
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							97.00%
Common stock shares issued pursuant to the terms of the Exchange Offer (in shares)							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							121,100,000	96,100,000	82,800,000
Gain (loss) on early extinguishment of debt	(2,742,000)	(1,300,000)	(6,485,000)	(42,796,000)		(1,300,000)	(41,500,000)						(2,700,000)	(2,500,000)	(2,400,000)
Unamortized issuance costs written off due to extinguishment of debt						140,000	4,700,000						300,000	400,000	400,000
Repurchase of unsecured convertible notes, aggregate cash price			221,439,000	12,755,000		12,800,000							122,800,000	98,600,000	84,600,000
Settlement consideration allocated to the reacquisition of the equity component			(2,981,000)			5,200,000							500,000	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	84,484,000		84,484,000		295,293,000				234,000		230,000		84,250,000		295,063,000
Unamortized discount									$ 16,000		$ 20,000		$ 551,000		$ 6,871,000