Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Earnings Per Share [Abstract]
Net income (loss)	$ (4,975)	$ 32,332	$ 66,533	$ 3,440	$ (7,997)	$ 21,375	$ 64,943	$ 3,651	$ 97,330	$ 81,972	$ 71,993
Weighted average shares outstanding [Abstract]
Basic (in shares)									46,282	46,937	48,158
Effect of dilutive securities [Abstract]
Stock options and employee stock purchase plan (in shares)									1,248	1,121	829
Diluted (in shares)									47,530	48,058	48,987
Basic (in dollars per share)	$ (0.11)	$ 0.70	$ 1.43	$ 0.07	$ (0.17)	$ 0.46	$ 1.38	$ 0.08	$ 2.10	$ 1.75	$ 1.49
Diluted (in dollars per share)	$ (0.11)	$ 0.68	$ 1.39	$ 0.07	$ (0.17)	$ 0.45	$ 1.34	$ 0.08	$ 2.05	$ 1.71	$ 1.47
Anti-dilutive stock options whose exercise prices were higher than the common stock's average market price during the period (in shares)									0	417	1,381