Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings (loss) per share:
Net income (loss)	$ (10,115)	$ 24,169	$ 58,577	$ (638)	$ (11,805)	$ 22,784	$ 52,770	$ (6,111)	$ 71,993	$ 57,638	$ 19,202
Weighted average shares outstanding [Abstract]
Basic (in shares)									48,158	49,469	48,649
Effect of dilutive securities [Abstract]
Stock options and employee stock purchase plan (in shares)									829	692	400
Diluted (in shares)									48,987	50,161	49,049
Basic (in dollars per share)	$ (0.21)	$ 0.50	$ 1.21	$ (0.01)	$ (0.24)	$ 0.46	$ 1.07	$ (0.12)	$ 1.49	$ 1.17	$ 0.39
Diluted (in dollars per share)	$ (0.21)	$ 0.50	$ 1.19	$ (0.01)	$ (0.24)	$ 0.45	$ 1.05	$ (0.12)	$ 1.47	$ 1.15	$ 0.39
Anti-dilutive stock options whose exercise prices were higher than the common stock's average market price during the period (in shares)									1,381	1,964	3,355