Stockholders' Equity and EPS - Reconciliation of Basic EPS to Diluted EPS (Detail) - 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
Numerator:
Net income available to common stockholders	$ 237,798	$ 123,193	$ 432,759	$ 224,676
Denominator:
Weighted average common shares outstanding-basic (in shares)	53,064	52,537	52,974	52,441
Denominator for diluted calculation (in shares)	53,776	53,194	53,732	53,180
Earnings per common share:
Basic (dollars per share)	$ 4.48	$ 2.34	$ 8.17	$ 4.28
Diluted (dollars per share)	$ 4.42	$ 2.32	$ 8.05	$ 4.22
Stock options and ESPP
Denominator:
Weighted average effect of dilutive securities (in shares)	400	368	408	397
Restricted stock units
Denominator:
Weighted average effect of dilutive securities (in shares)	312	289	350	342

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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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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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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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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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