Computation of Basic and Diluted Net Profit Per Share Attributable to Common Stock (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Disclosure Computation Of Basic And Diluted Net Profit Per Share Attributable To Common Stock [Abstract]
Net income	$ 36,177	$ 32,444
Weighted-average common shares outstanding, basic	80,459	81,120
Dilutive effect of potential common stock	1,365	1,697
Total shares, diluted	81,824	82,817
Net income per share, basic	$ 0.45	$ 0.40
Net income per share, diluted	$ 0.44	$ 0.39