Stockholders' Equity and EPS - Reconciliation of Basic EPS to Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income available to common stockholders	$ 288,732	$ 194,961
Denominator:
Weighted average common shares outstanding-basic (in shares)	52,587	52,883
Weighted average common shares outstanding—diluted (in shares)	53,109	53,685
Earnings per common share:
Basic (dollars per share)	$ 5.49	$ 3.69
Diluted (dollars per share)	$ 5.44	$ 3.63
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities (in shares)	297	420
Restricted stock units and awards
Denominator:
Weighted average effect of dilutive securities (in shares)	225	382

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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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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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