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Description of offering period resets.

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Aggregate number of common shares reserved for future issuance.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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Unrecognized cost of unvested options awarded to employees as compensation.

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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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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.

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The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Number of share instruments newly issued under a share-based compensation plan.

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Percentage of vesting of share-based compensation awards.

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Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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Purchase price of common stock expressed as a percentage of its fair value.

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Stock-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		3 Months Ended			6 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Jul. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Jul. 31, 2018	Jul. 31, 2017	Jan. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate intrinsic value of exercised options						$ 22.5	$ 11.2
Aggregate estimated fair value of stock options granted to employees vested						3.9	$ 4.8
Unrecognized stock-based compensation expense related to stock option			$ 11.1			$ 11.1
Remaining weighted-average period						2 years 8 months 26 days
Stock options forfeited						408,689
Performance-Based Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense related to stock option			4.7			$ 4.7
Remaining weighted-average period						3 years 6 months 14 days
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining weighted-average period						3 years 1 month 9 days
Unrecognized stock-based compensation expense			$ 261.8			$ 261.8
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Dividend yield			0.00%		0.00%	0.00%	0.00%
Class A Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average grant date fair value of options granted to employees						$ 8.23	$ 6.82
2015 Equity Incentive Plan \| Performance-Based Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted	650,000	475,000
Stock options forfeited				250,000
2015 Equity Incentive Plan \| Class A Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares common stock reserved for issuance			19,348,110			19,348,110		12,200,000
2015 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares common stock reserved for issuance			2,571,580			2,571,580
Percentage of eligible compensation allowed to employees to purchase shares at a discount			15.00%			15.00%
Description of offering period						The 2015 ESPP provides for 24-month offering periods beginning March 16 and September 16 of each year, and each offering period consists of four six-month purchase periods.
Purchase price of common stock, percentage						85.00%
Description of offering period resets						the offering period resets and the new lower price becomes the new offering price for a new 24 month offering period.
Unrecognized stock-based compensation expense			$ 11.8			$ 11.8
Dividend yield			0.00%		0.00%	0.00%	0.00%
2015 Employee Stock Purchase Plan \| Class A Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares common stock reserved for issuance								2,500,000
Vesting Commencement Per Year \| 2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options and restricted stock units vesting percentage						25.00%
Vesting Commencement Per Year \| 2015 Equity Incentive Plan \| Performance-Based Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options and restricted stock units vesting percentage	25.00%
Vesting Commencement Per Month
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options and restricted stock units vesting percentage						2.08%
Vesting Commencement Per Month \| 2015 Equity Incentive Plan \| Performance-Based Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options and restricted stock units vesting percentage	2.08%
Vesting Commencement Per Quarter
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options and restricted stock units vesting percentage						6.25%