Earnings Per Share - Summary of Basic and Diluted EPS (Detail) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]
Net (loss) income attributable to common and participating preferred stockholders	$ (6.3)	$ 49.0	$ (87.6)	$ (39.0)	$ (202.3)	$ 91.6	$ (45.5)	$ 62.0	$ (83.9)	$ (94.2)	$ 29.9
Participating shares at end of period:
Common shares outstanding									196,878	138,876	139,357
Preferred shares (as-converted basis)									0	61,987	62,839
Total									196,878	200,863	202,196
Percentage of income loss allocated to:
Common shares									100.00%	100.00%	68.90%
Preferred shares (a)									0.00%	0.00%	31.10%
Net (loss) income attributable to common shares - basic									(83.9)	(94.2)	20.6
Net (loss) income attributable to common shares - diluted									$ (83.9)	$ (94.2)	$ 20.6
Weighted-average common shares outstanding - basic									162,941	139,856	139,356
Dilutive effect of unvested restricted stock and restricted stock units											381
Dilutive effect of stock options											81
Weighted-average shares outstanding - diluted									162,941	139,856	139,818
Net (loss) income per common share attributable to controlling interest:
Basic (in dollars per share)	$ (0.03)	$ 0.28	$ (0.63)	$ (0.28)	$ (1.45)	$ 0.45	$ (0.33)	$ 0.31	$ (0.51)	$ (0.67)	$ 0.15
Earnings Per Share, Diluted	$ (0.03)	$ 0.28	$ (0.63)	$ (0.28)	$ (1.45)	$ 0.25	$ (0.33)	$ 0.03	$ (0.51)	$ (0.67)	$ 0.15