Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share [Abstract]
Net income (loss)	$ (7,997)	$ 21,375	$ 64,943	$ 3,651	$ (10,115)	$ 24,169	$ 58,577	$ (638)	$ 81,972	$ 71,993	$ 57,638
Weighted average shares outstanding [Abstract]
Basic (in shares)									46,937	48,158	49,469
Effect of dilutive securities [Abstract]
Stock options and employee stock purchase plan (in shares)									1,121	829	692
Diluted (in shares)									48,058	48,987	50,161
Basic (in dollars per share)	$ (0.17)	$ 0.46	$ 1.38	$ 0.08	$ (0.21)	$ 0.50	$ 1.21	$ (0.01)	$ 1.75	$ 1.49	$ 1.17
Diluted (in dollars per share)	$ (0.17)	$ 0.45	$ 1.34	$ 0.08	$ (0.21)	$ 0.50	$ 1.19	$ (0.01)	$ 1.71	$ 1.47	$ 1.15
Anti-dilutive stock options whose exercise prices were higher than the common stock's average market price during the period (in shares)									417	1,381	1,964