Common Stock and Stockholders' Equity (Assumptions Used for Periods Presented) (Detail) - $ / shares	12 Months Ended
Jan. 31, 2019	Jan. 31, 2018	Jan. 31, 2017
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, Minimum	31.50%
Expected volatility, Maximum	34.30%
Risk-free interest rate, Minimum	2.10%
Risk-free interest rate, Maximum	2.72%
Dividend yield, percentage	0.00%
Stock Options \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	11 days
Stock Options \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	2 years 5 months 2 days
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, Minimum	30.90%	25.30%	34.50%
Expected volatility, Maximum	41.70%	32.00%	44.50%
Expected term (in years)	6 months	6 months	6 months
Risk-free interest rate, Minimum	2.09%	1.11%	0.53%
Risk-free interest rate, Maximum	2.50%	1.45%	0.91%
Dividend yield, percentage	0.00%	0.00%	0.00%
Offering Period 1 \| ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value per share, dollars per share	$ 126.29	$ 98.39	$ 79.30
Offering Period 2 \| ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value per share, dollars per share	$ 167.80	$ 100.52	$ 69.00

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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- Definition

The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

No definition available.

+ Details

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- Definition

The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

No definition available.

+ Details

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- Definition

The maximum risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

No definition available.

+ Details

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- Definition

The minimum risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Grant Date Fair Value

+ References

No definition available.

+ Details

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