Stockholders' Equity and EPS - Reconciliation of Basic EPS to Diluted EPS (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
Numerator:
Net income available to common stockholders	$ 317,987	$ 237,798	$ 606,719	$ 432,759
Denominator:
Weighted average common shares outstanding-basic (in shares)	51,955	53,064	52,269	52,974
Weighted average common shares outstanding—diluted (in shares)	52,336	53,776	52,715	53,732
Earnings per common share:
Basic (dollars per share)	$ 6.12	$ 4.48	$ 11.61	$ 8.17
Diluted (dollars per share)	$ 6.08	$ 4.42	$ 11.51	$ 8.05
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities (in shares)	235	400	254	408
Restricted stock units and awards
Denominator:
Weighted average effect of dilutive securities (in shares)	146	312	192	350

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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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