Stock-based Compensation - Nonvested Stock Awards (Details)	6 Months Ended
	Mar. 31, 2018 $ / shares shares
Restricted Stock
Shares
Beginning of period (in shares) | shares	1,540,947
Granted (in shares) | shares	376,250
Vested (in shares) | shares	(225,608)
Forfeited (in shares) | shares	(203,743)
End of period (in shares) | shares	1,487,846
Weighted Average Grant Date Fair Value
Beginning of period (in dollars per share) | $ / shares	$ 16.14
Granted (in dollars per share) | $ / shares	21.17
Vested (in dollars per share) | $ / shares	14.01
Forfeited (in dollars per share) | $ / shares	18.76
End of period (in dollars per share) | $ / shares	$ 17.38
Performance-Based Restricted Stock Awards
Shares
Beginning of period (in shares) | shares	668,766
Granted (in shares) | shares	165,085
Vested (in shares) | shares	0
Forfeited (in shares) | shares	(188,058)
End of period (in shares) | shares	645,793
Weighted Average Grant Date Fair Value
Beginning of period (in dollars per share) | $ / shares	$ 15.72
Granted (in dollars per share) | $ / shares	22.40
Vested (in dollars per share) | $ / shares	0.00
Forfeited (in dollars per share) | $ / shares	18.96
End of period (in dollars per share) | $ / shares	$ 16.48
Time Based Restricted Stock Awards
Shares
Beginning of period (in shares) | shares	872,181
Granted (in shares) | shares	211,165
Vested (in shares) | shares	(225,608)
Forfeited (in shares) | shares	(15,685)
End of period (in shares) | shares	842,053
Weighted Average Grant Date Fair Value
Beginning of period (in dollars per share) | $ / shares	$ 16.47
Granted (in dollars per share) | $ / shares	20.20
Vested (in dollars per share) | $ / shares	14.01
Forfeited (in dollars per share) | $ / shares	16.35
End of period (in dollars per share) | $ / shares	$ 18.07