Earnings Per Share (Computation of Basic and Diluted 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 Per Share
Net income/(loss)	$ 27,016	$ 22,304	$ 3,416	$ 31,398	$ 41,394	$ (84,336)	$ 21,109	$ 23,740
Average shares used in basic computation									52,418	[1]	48,723	[1]	42,445	[1]
Dilutive effect of stock options and units									10,640		8,949		5,996
Weighted Average Number Diluted Shares Outstanding Adjustment, Total									63,058		57,672		48,441
Basic									$ 1.61	[1]	$ 0.04	[1]	$ 1.79	[1]
Diluted									$ 1.33		$ 0.03		$ 1.56

[1] Diluted earnings per share is computed on the basis of the weighted average number of shares of common stock plus the effect of dilutive potential common shares outstanding during the period using the treasury stock method. Diluted earnings per share include stock options and units.