Earnings Per Share Earnings Per Share (Detail 1) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 56.1	$ 54.6	$ 70.7	$ 89.3
Earnings allocated to participating securities	0.0	(0.1)	0.0	(0.2)
Earnings available to common shareholders	$ 56.1	$ 54.5	$ 70.7	$ 89.1
Basic weighted-average common shares outstanding (in shares)	73,118,295	78,007,479	73,593,439	78,433,035
Dilutive stock options and other equity-based compensation awards (in shares)	743,045	1,102,424	766,421	1,103,796
Participating restricted stock (in shares)	0	(115,163)	0	(112,237)
Diluted weighted-average common shares outstanding (in shares)	73,861,340	78,994,740	74,359,860	79,424,594