Stock Incentive Plan - Stock Options Activity (Details) - Stock Options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Shares (in thousands)
Outstanding, beginning balance (shares)	5,158	6,664	6,746
Granted (shares)	185	171	72
Exercised (shares)	(2,728)	(1,629)	(18)
Forfeited (shares)	(10)	(48)	(136)
Outstanding, ending balance (shares)	2,605	5,158	6,664
Vested (shares)	1,915
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 8.30	$ 7.76	$ 7.70
Granted (in dollars per share)	20.01	21.45	15.79
Exercised (in dollars per share)	7.55	7.47	7.95
Forfeited (in dollars per share)	8.28	9.02	8.84
Outstanding, ending balance (in dollars per share)	9.91	8.30	7.76
Vested (in dollars per share)	8.41
Stock Options, Additional Disclosures
Weighted average grant date fair value of options granted	$ 8.33	$ 8.84	$ 7.63
Exercised, aggregate intrinsic value	$ 42,512	$ 25,757	$ 43
Outstanding, aggregate intrinsic value	43,302
Vested, aggregate intrinsic value	$ 34,697
Outstanding, weighted average remaining contractual term (in years)	5 years 7 months 10 days
Vested, weighted average remaining contractual term (in years)	5 years 15 days