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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 109,315	$ 80,316	$ 205,482	$ 158,837
Basic:
Weighted average shares (in shares)	121,307	124,617	121,619	125,074
Diluted:
Weighted average shares (in shares)	121,307	124,617	121,619	125,074
Dilutive effect of potential common shares (in shares)	961	487	957	684
Diluted weighted average shares (in shares)	122,268	125,104	122,576	125,758
Net income per share:
Basic (in usd per share)	$ 0.90	$ 0.64	$ 1.69	$ 1.27
Diluted (in usd per share)	$ 0.89	$ 0.64	$ 1.68	$ 1.26

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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