Net Income Per Share - Calculation of Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Net income	$ 96,725	$ 85,184	$ 264,353	$ 221,875
Basic:
Weighted average shares	131,285	134,054	132,280	134,690
Diluted:
Weighted average shares	131,285	134,054	132,280	134,690
Dilutive effect of potential common shares	1,203	1,312	1,156	1,050
Diluted weighted average shares	132,488	135,366	133,436	135,740
Net income per share:
Basic (in usd per share)	$ 0.74	$ 0.64	$ 2.00	$ 1.65
Diluted (in usd per share)	$ 0.73	$ 0.63	$ 1.98	$ 1.63