Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Earnings Per Share Reconciliation [Line Items]
Net income	$ 17,156	$ 23,726	$ 58,431	$ 43,670
Denominator for basic earnings per share - weighted-average number of common shares outstanding during the period	53,660,000	54,087,000	53,712,000	54,361,000
Incremental common shares attributable to exercise of outstanding dilutive options	445,000	360,000	460,000	284,000
Incremental common shares attributable to outstanding restricted shares and restricted stock units	160,000	164,000	215,000	223,000
Denominator for diluted earnings per share	54,265,000	54,611,000	54,387,000	54,868,000
Basic earnings per share	$ 0.32	$ 0.44	$ 1.09	$ 0.80
Diluted earnings per share	$ 0.32	$ 0.43	$ 1.07	$ 0.80
Anti-dilutive options to purchase common shares	800,000	2,000,000	700,000	2,300,000