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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 115,242	$ 84,700	$ 320,724	$ 243,537
Basic:
Weighted average shares (in shares)	120,115	123,569	121,112	124,567
Diluted:
Weighted average shares (in shares)	120,115	123,569	121,112	124,567
Dilutive effect of potential common shares (in shares)	1,328	631	1,081	666
Diluted weighted average shares (in shares)	121,443	124,200	122,193	125,233
Net income per share:
Basic (in usd per share)	$ 0.96	$ 0.69	$ 2.65	$ 1.96
Diluted (in usd per share)	$ 0.95	$ 0.68	$ 2.62	$ 1.94

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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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