Equity-Based Compensation - Black Scholes Options - Pricing Model (Details) - Stock Options - $ / shares	9 Months Ended
Jan. 31, 2018	Jan. 31, 2017
Fair Value Assumptions
Volatility (as a percent)	30.86%	41.10%
Expected life (years)	6 years	6 years
Risk-free interest rate (as a percent)	2.18%	1.53%
Weighted average grant date fair value (in dollars per share)	$ 12.81	$ 9.53

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- Definition

Period the instrument, asset or liability is expected to be outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Risk-free interest rate assumption used in valuing an instrument.

+ References

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Weighted average expected volatility of stock price.

+ References

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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