Convertible Debt (Details) (USD $)	3 Months Ended		9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended		1 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Apr. 30, 2007 Due 2017 [Member]	Sep. 30, 2011 Due 2017 [Member]	Sep. 30, 2010 Due 2017 [Member]	Sep. 30, 2011 Due 2017 [Member]	Sep. 30, 2010 Due 2017 [Member]	Sep. 30, 2011 Due 2013 [Member]	Nov. 30, 2010 Due 2013 [Member]	Mar. 31, 2006 Due 2013 [Member]	Sep. 30, 2011 Due 2013 [Member]	Sep. 30, 2010 Due 2013 [Member]	Sep. 30, 2011 Due 2013 [Member]	Sep. 30, 2010 Due 2013 [Member]
Debt Conversion [Line Items]
Debt instrument, face amount					$ 324,900,000							$ 172,500,000
Debt instrument, interest rate, stated percentage, per annum					1.875%							2.50%
Debt instrument, convertible, conversion price, per share					$ 20.36							$ 16.58
Payments of debt issuance costs					8,500,000							5,500,000
Amortization expense						200,000	200,000	600,000	600,000				100,000	200,000	200,000	600,000
Debt instrument, convertible, interest expense	1,800,000	2,600,000	5,500,000	7,800,000						1,100,000	7,200,000
Convertible notes										29,200,000	119,600,000
Notes converted, number of shares										1,760,178	7,213,379
Convertible cash premium paid to holder for agreeing to convert										800,000	6,500,000
Deferred offering costs										$ 200,000	$ 1,300,000		$ 200,000		$ 200,000
Maturity date of convertible debt					April 23, 2017							March 29, 2013