Acquisitions - Other insignficant acquisitions (Detail) - USD ($) $ in Millions				3 Months Ended	12 Months Ended
	Jan. 16, 2015	Dec. 31, 2014	Oct. 01, 2014	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2014	Sep. 30, 2015	Oct. 31, 2014	Oct. 06, 2014	Jan. 02, 2014	Sep. 30, 2013	Feb. 14, 2013
Business Acquisition [Line Items]
Estimated Income tax rate					35.00%	35.00%
Oil and natural gas properties
Goodwill (Note 12)					$ 2,478.4		$ 2,487.4				$ 1,524.8
The Liquid Fence Company [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents										$ 0.1
Payments to Acquire Businesses, Gross						$ 24.8
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Inventory										2.1
Goodwill (Note 12)			$ 26.9							26.9
Oil and natural gas properties
Property, plant and equipment										0.1
Goodwill (Note 12)										7.1
Business Combination Purchase Consideration Transferred						35.8
Business Combination, Consideration Transferred, Liabilities Incurred						$ 9.5
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables										1.1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable										1.6
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net										$ 35.8
Compass Production Partners [Member]
Oil and natural gas properties
Business Acquisition Percentage Of Equity Interests Acquired									25.50%
Ability Re [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Insurance Reserves								$ (346.9)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents								8.5
Payments to Acquire Businesses, Gross				$ 17.9
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Funds Withheld								359.5
Oil and natural gas properties
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets								368.0
Business Combination Purchase Consideration Transferred				19.2
Business Combination, Consideration Transferred, Liabilities Incurred				2.8
Business Combination, Consideration Transferred, Other				(1.5)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other								(1.9)
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net								$ 19.2
Tell Manufacturing [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents			1.1
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Inventory			7.2
Property, plant and equipment			1.5
Goodwill (Note 12)			12.5
Oil and natural gas properties
Property, plant and equipment			12.5
Goodwill (Note 12)			7.1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets			0.6
Business Combination Purchase Consideration Transferred				30.3
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables			6.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable			(5.7)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net			30.3
European IAMS and Eukanuba [Member]
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Inventory		$ 16.3		16.3
Goodwill (Note 12)		39.6		39.6
Oil and natural gas properties
Property, plant and equipment		58.3		58.3
Goodwill (Note 12)		4.0		4.0
Business Combination Purchase Consideration Transferred				115.7
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other		(2.7)		(2.7)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets		2.9		2.9
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable		2.7		2.7
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net		115.7		115.7
Salix Animal Health LLC [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	$ 0.5
Payments to Acquire Businesses, Gross					146.8
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Inventory	17.0
Goodwill (Note 12)	55.5
Oil and natural gas properties
Property, plant and equipment	1.2
Goodwill (Note 12)	71.5
Business Combination Purchase Consideration Transferred					148.3
Business Combination, Consideration Transferred, Liabilities Incurred					$ 1.5
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables	10.7
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other	(2.1)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets	2.5
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accounts Payable	8.5
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net	148.3
Compass | EXCO [Member]
Oil and natural gas properties
Business Acquisition Percentage Of Equity Interests Acquired												24.50%
Compass | HGI
Oil and natural gas properties
Business Acquisition Percentage Of Equity Interests Acquired					74.40%
Tradenames Trademarks And Patents [Member] | The Liquid Fence Company [Member]
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill (Note 12)			5.1
Tradenames Trademarks And Patents [Member] | Tell Manufacturing [Member]
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill (Note 12)			4.0
Tradenames Trademarks And Patents [Member] | European IAMS and Eukanuba [Member]
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill (Note 12)		25.5		25.5
Tradenames Trademarks And Patents [Member] | Salix Animal Health LLC [Member]
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill (Note 12)	17.0
Technology-Based Intangible Assets [Member] | The Liquid Fence Company [Member]
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill (Note 12)			$ 20.5
Oil and natural gas properties
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			17 years
Technology-Based Intangible Assets [Member] | European IAMS and Eukanuba [Member]
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill (Note 12)		$ 3.6		$ 3.6
Oil and natural gas properties
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life					8 years
Technology-Based Intangible Assets [Member] | Salix Animal Health LLC [Member]
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill (Note 12)	$ 2.1
Oil and natural gas properties
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	17 years
Licensing Agreements [Member] | Salix Animal Health LLC [Member]
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill (Note 12)	$ 1.0
Oil and natural gas properties
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	13 years
Customer Relationships [Member]
Business Acquisition [Line Items]
Estimated Income tax rate					38.00%
Customer Relationships [Member] | The Liquid Fence Company [Member]
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill (Note 12)			$ 1.3
Oil and natural gas properties
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life				15 years
Customer Relationships [Member] | Tell Manufacturing [Member]
Business Acquisition [Line Items]
Estimated Income tax rate			38.00%
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill (Note 12)			$ 8.5
Oil and natural gas properties
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life				13 years
Customer Relationships [Member] | European IAMS and Eukanuba [Member]
Business Acquisition [Line Items]
Estimated Income tax rate		25.00%
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill (Note 12)		$ 10.5		$ 10.5
Oil and natural gas properties
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life					15 years
Customer Relationships [Member] | Salix Animal Health LLC [Member]
Business Acquisition [Line Items]
Estimated Income tax rate	38.00%
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill (Note 12)	$ 35.4
Oil and natural gas properties
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	13 years
Minimum [Member] | Customer Relationships [Member]
Oil and natural gas properties
Amortization period, intangible asset					2 years
Maximum | Customer Relationships [Member]
Oil and natural gas properties
Amortization period, intangible asset					20 years