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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net income	$ 114,612	$ 109,315	$ 224,410	$ 205,482
Basic:
Weighted average shares (in shares)	116,381	121,307	116,722	121,619
Diluted:
Weighted average shares (in shares)	116,381	121,307	116,722	121,619
Dilutive effect of potential common shares (in shares)	607	961	753	957
Diluted weighted average shares (in shares)	116,988	122,268	117,475	122,576
Net income per share:
Basic (in usd per share)	$ 0.98	$ 0.90	$ 1.92	$ 1.69
Diluted (in usd per share)	$ 0.98	$ 0.89	$ 1.91	$ 1.68