Stockholders' Equity - RSU activity (Details)	6 Months Ended
	Mar. 31, 2020 $ / shares shares
Service-Based RSUs
Number of Shares
Balance unvested at the beginning of the period (in shares) | shares	422,094
Granted (in shares) | shares	348,055
Vested (in shares) | shares	(158,931)
Forfeited (in shares) | shares	(43,913)
Balance unvested at the end of the period (in shares) | shares	567,305
Weighted Average Grant-Date Fair Value
Balance unvested at the beginning of the period (in dollars per share) | $ / shares	$ 58.84
Granted (in dollars per share) | $ / shares	59.81
Vested (in dollars per share) | $ / shares	55.80
Forfeited (in dollars per share) | $ / shares	59.94
Balance unvested at the end of the period (in dollars per share) | $ / shares	$ 60.20
Performance-based RSUs
Number of Shares
Balance unvested at the beginning of the period (in shares) | shares	315,262
Forfeited (in shares) | shares	(177,492)
Balance unvested at the end of the period (in shares) | shares	137,770
Weighted Average Grant-Date Fair Value
Balance unvested at the beginning of the period (in dollars per share) | $ / shares	$ 55.67
Forfeited (in dollars per share) | $ / shares	47.57
Balance unvested at the end of the period (in dollars per share) | $ / shares	$ 61.40
Performance and market-based RSUs
Number of Shares
Balance unvested at the beginning of the period (in shares) | shares	227,402
Granted (in shares) | shares	346,826
Forfeited (in shares) | shares	(46,896)
Balance unvested at the end of the period (in shares) | shares	527,332
Weighted Average Grant-Date Fair Value
Balance unvested at the beginning of the period (in dollars per share) | $ / shares	$ 66.77
Granted (in dollars per share) | $ / shares	52.51
Forfeited (in dollars per share) | $ / shares	59.24
Balance unvested at the end of the period (in dollars per share) | $ / shares	$ 58.06