Stock-Based Compensation - Stock Option Activity (Details) - Stock options - USD ($)	9 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Sep. 30, 2018
Stock options, Shares Subject to Options
Outstanding at the beginning of the period (in shares)		2,593,310
Granted (in shares)		112,232
Exercised (in shares)		(465,701)
Forfeited /Expired (in shares)		(85,340)
Outstanding at the end of the period (in shares)		2,154,501
Exercisable at the end of the period (in shares)		1,544,782
Exercisable and expected to vest at the end of the period (in shares)		2,103,772
Stock options, Weighted Average Option Price
Outstanding at the beginning of the period (in dollars per share)		$ 21.41
Granted (in dollars per share)		44.17
Exercised (in dollars per share)		19.27
Forfeited (in dollars per share)		22.35
Outstanding at the end of the period (in dollars per share)		23.03
Exercisable at the end of the period (in dollars per share)		20.49
Exercisable and expected to vest at the end of the period (in dollars per share)		$ 22.87
Stock options, additional information
Weighted Average Remaining Contractual Term, Outstanding		5 years 1 month 6 days
Weighted Average Remaining Contractual Term, Exercisable		4 years 9 months 18 days
Weighted Average Remaining Contractual Term, Exercisable and expected to vest		5 years 1 month 6 days
Aggregate Intrinsic Value, Outstanding		$ 45.2
Aggregate Intrinsic Value, Exercisable		36.2
Aggregate Intrinsic Value, Exercisable and expected to vest		$ 44.4
Fair value of the Company's common stock price (in dollars per share)		$ 43.93
Weighted average fair values of options granted (in dollars per share)	$ 11.16		$ 9.50
Intrinsic value of options exercised		$ 10,300,000	$ 7,200,000
Minimum
Stock-Based Compensation
Vesting period		3 years
Maximum
Stock-Based Compensation
Vesting period		4 years