Net Income Per Share - Calculation of Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share [Abstract]
Net income	$ 89,706	$ 75,140	$ 167,628	$ 136,691
Basic:
Weighted average shares	132,499	134,699	132,786	135,014
Diluted:
Weighted average shares	132,499	134,699	132,786	135,014
Dilutive effect of potential common shares	1,054	1,009	1,132	918
Diluted weighted average shares	133,553	135,708	133,918	135,932
Net Income Per Share:
Basic (in usd per share)	$ 0.68	$ 0.56	$ 1.26	$ 1.01
Diluted (in usd per share)	$ 0.67	$ 0.55	$ 1.25	$ 1.01