Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Numerator
Net income	$ 61,920	$ 69,002
Denominator
Weighted-average shares outstanding, basic	71,572	73,054
Effect of dilutive stock awards (in shares)	204	316
Adjusted weighted-average shares outstanding, diluted	71,777	73,370
Net income per share
Basic earnings per common share (in dollars per share)	$ 0.87	$ 0.94
Diluted earnings per common share (in dollars per share)	$ 0.86	$ 0.94
Anti-dilutive weighted-average shares	85	27

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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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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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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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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