Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share Reconciliation [Line Items]
Net income	$ 23,726	$ 19,314	$ 43,670	$ 38,492
Denominator for basic earnings per share - weighted-average number of common shares outstanding during the period	54,087,000	55,814,000	54,361,000	56,750,000
Denominator for diluted earnings per share	54,611,000	56,028,000	54,868,000	57,054,000
Basic earnings per share	$ 0.44	$ 0.35	$ 0.80	$ 0.68
Diluted earnings per share	$ 0.43	$ 0.34	$ 0.80	$ 0.67
Stock Options [Member]
Earnings Per Share Reconciliation [Line Items]
Incremental common shares attributable to outstanding stock-based awards	360,000	131,000	284,000	155,000
Options to purchase of common shares	2,000,000	3,800,000	2,300,000	3,700,000
Restricted Shares, Restricted Stock Units And Phantom Stock [Member]
Earnings Per Share Reconciliation [Line Items]
Incremental common shares attributable to outstanding stock-based awards	164,000	83,000	223,000	149,000