Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share Reconciliation [Line Items]
Net income	$ 19,314	$ 19,867	$ 38,492	$ 49,081
Denominator for basic earnings per share - weighted-average number of common shares outstanding during the period	55,814,000	58,615,000	56,750,000	59,889,000
Denominator for diluted earnings per share	56,028,000	58,879,000	57,054,000	60,391,000
Basic earnings per share	$ 0.35	$ 0.34	$ 0.68	$ 0.82
Diluted earnings per share	$ 0.34	$ 0.34	$ 0.67	$ 0.81
Options to purchase of common shares	3,800,000	3,800,000	3,700,000	3,500,000
Stock Options [Member]
Earnings Per Share Reconciliation [Line Items]
Incremental common shares attributable to exercise of outstanding dilutive options	131,000	177,000	155,000	298,000
Restricted Shares, Restricted Stock Units And Phantom Stock [Member]
Earnings Per Share Reconciliation [Line Items]
Incremental common shares attributable to exercise of outstanding dilutive options	83,000	87,000	149,000	204,000