Net Income per Share (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Debt Instruments [Abstract]
Outstanding stock options excluded from net income per diluted share calculations (in shares)	200,000		200,000		200,000		400,000
Numerator [Abstract]
Net income	$ 45,916,000		$ 40,189,000		$ 160,447,000		$ 116,334,000
Add back	841,000	[1]	987,000	[1]	3,391,000	[1]	3,982,000	[1]
Income used to compute net income per diluted share	46,757,000		41,176,000		163,838,000		120,316,000
Denominator [Abstract]
Total weighted-average shares used to compute basic income per share (in shares)	139,680,000		127,479,000		139,665,000		122,209,000
Effect of dilutive stock options (in shares)	13,000		10,000		13,000		9,000
Restricted stock outstanding (in shares)	12,000		106,000		21,000		99,000
Shares used to compute income per diluted share (in shares)	167,019,000		172,217,000		186,756,000		178,448,000
Net Income per Basic Share [Abstract]
Net income per basic share (dollars per share)	$ 0.33		$ 0.32		$ 1.15		$ 0.95
Net income per diluted share (dollars per share)	$ 0.28		$ 0.24		$ 0.88		$ 0.67
Estimated tax on convertible note interest expense (1)	$ 500,000		$ 500,000		$ 1,800,000		$ 2,100,000
Due 2012 [Member]
Debt Instruments [Abstract]
Interest rate of convertible note (in hundredths)	2.00%		2.00%		2.00%		2.00%
Assumed conversion of debt notes (in shares)	0		32,050		19,743		32,050
Due February 2015 [Member]
Debt Instruments [Abstract]
Interest rate of convertible note (in hundredths)	2.875%		2.875%		2.875%		2.875%
Conversion ratio in shares (in shares)					151.713
Conversion price (dollars per share)	$ 6.59				$ 6.59
Assumed conversion of debt notes (in shares)	27,314				27,314		0
Due 2023 [Member]
Debt Instruments [Abstract]
Interest rate of convertible note (in hundredths)	2.75%		2.75%		2.75%		2.75%
Assumed conversion of debt notes (in shares)	0		12,572		0		24,081
Due May 2015 [Member]
Debt Instruments [Abstract]
Interest rate of convertible note (in hundredths)	3.75%				3.75%
Conversion ratio in shares (in shares)					132.6682
Conversion price (dollars per share)	$ 7.54				$ 7.54

[1] Interest expense for convertible notes, net of estimated tax of $0.5 million for each of the three months ended September 30, 2011 and 2010, respectively, and $1.8 million and $2.1 million for the nine months ended September 30, 2011 and 2010, respectively (see Note 9)