Acquisition of NHUMO - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Nov. 30, 2013	Dec. 31, 2014	Sep. 30, 2015	Sep. 30, 2014
Business Acquisition [Line Items]
Gain on acquisition						$ 29.0
Customer Relationships [Member]
Business Acquisition [Line Items]
Intangible asset					$ 63.0
Amortized period					20 years
NHUMO [Member]
Business Acquisition [Line Items]
Percentage acquisition			60.00%
Ownership percentage prior to acquisition			40.00%
Cash paid on acquisition		$ 80.0	$ 80.0
Dividend percentage of preferred stock			6.00%
Preferred stock issued on acquisition			$ 25.0
Dividend payment	$ 1.5
Dividend received from NHUMO			14.0
Dividends payable					$ 1.5
Gain on acquisition				$ 29.0
Previously held equity interest in NHUMO		$ 54.0				54.0
NHUMO [Member] | Selling and Administrative Expenses [Member]
Business Acquisition [Line Items]
Acquisition costs incurred						$ 2.0
NHUMO [Member] | Carrying Value [Member]
Business Acquisition [Line Items]
Preferred stock redemption amount			$ 25.0