Share-Based Payments - Service-Based Awards and Stock Option Awards (Details) - Stock Option Awards $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares
Stock Options, Number of Outstanding Options
Outstanding at the beginning of the period (in shares) | shares	5,405
Granted (in shares) | shares	948
Exercised (in shares) | shares	(127)
Forfeited or expired (in shares) | shares	(849)
Outstanding at the end of the period (in shares) | shares	5,377	5,405
Exercisable at the end of the period (in shares) | shares	3,444
Stock Options, Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 23.04
Granted (in dollars per share) | $ / shares	18.14
Exercised (in dollars per share) | $ / shares	15.42
Forfeited or expired (in dollars per share) | $ / shares	25.32
Outstanding at the end of the period (in dollars per share) | $ / shares	21.99	$ 23.04
Exercisable at the end of the period (in dollars per share) | $ / shares	$ 23.68
Stock Options, Weighted Average Remaining Contractual Term
Weighted average remaining contractual term (in years)	6 years 3 months 18 days	5 years 4 months 24 days
Exercisable at March 31, 2019	4 years 10 months 24 days
Stock Options, Aggregate Intrinsic Value
Outstanding balance at period end (in dollars) | $	$ 2,924	$ 3,161
Exercisable at the end of the period (in dollars) | $	$ 1,773