Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		3 Months Ended		6 Months Ended
	May 31, 2019	Nov. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Jun. 30, 2019	May 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted (in shares)			0
Exercise price of stock options granted (in usd per share)			$ 0
Service period (in years)			5 years 6 months 18 days		5 years 9 months 18 days
Vested in period, value			$ 0.3	$ 0.3
Compensation cost related to nonvested stock options not yet recognized			$ 7.2
Unrecognized compensation cost related to nonvested stock option recognized over weighted average period, in years			1 year 8 months 12 days
Non-vested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost related to unvested restricted stock units			$ 31.0
Weighted average remaining vesting period			2 years 4 months 24 days
Grant of RSUs (in shares)			135,280
Fair value of shares granted (in usd per share)			$ 53.92
2017 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available to be issued (in shares)			4,595,825
2017 Equity Incentive Plan | Non-vested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Value of award at grant date		$ 7.7
Grant of RSUs (in shares)		255,930
Fair value of shares granted (in usd per share)		$ 30.06
2017 Equity Incentive Plan | Class A Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available to be issued (in shares)			7,129,569			6,421,442
2007 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available to be issued (in shares)						421,442
Number of shares available for grants (in shares)						0
Chief Executive Officer | 2017 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted (in shares)	700,000
Exercise price of stock options granted (in usd per share)	$ 33.98
Share price (in usd per share)	$ 84.63
Value of award at grant date	$ 9.5
Service period (in years)	2 years 7 months 6 days