Earnings Per Share (Details) (USD $) In Thousands, except Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Earnings Per Share Reconciliation [Line Items]
Net income	$ 19,867	$ 22,469	$ 49,081	$ 60,748
Denominator for basic earnings per share - weighted-average number of common shares outstanding during the period	58,615,000	61,712,000	59,889,000	62,597,000
Denominator for diluted earnings per share	58,879,000	62,103,000	60,391,000	63,162,000
Basic earnings per share	$ 0.34	$ 0.36	$ 0.82	$ 0.97
Diluted earnings per share	$ 0.34	$ 0.36	$ 0.81	$ 0.96
Options to purchase of common shares	3,800,000	3,700,000	3,500,000	2,900,000
Stock Options [Member]
Earnings Per Share Reconciliation [Line Items]
Incremental common shares attributable to exercise of outstanding dilutive options	177,000	305,000	298,000	474,000
Restricted Shares, Restricted Stock Units And Phantom Stock [Member]
Earnings Per Share Reconciliation [Line Items]
Incremental common shares attributable to outstanding restricted shares, restricted stock units and phantom stock	87,000	86,000	204,000	91,000