Summary of Significant Accounting Policies (Details) - USD ($)	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Accounting Policies [Abstract]
Ownership interests in certain subsidiaries under equity method investment, maximum	100.00%
Voting rights in investment businesses not traded publicly accounted for under the cost method, maximum	20.00%
Maturity period of highly liquid investments (in months)	3 months
Construction escrow agreement restricted cash	$ 14,300,000
Accumulated impairment losses	0	$ 0
Provision for goodwill or other intangible asset impairments	0	0	$ 0
Provisions for impairments	0	0	0
Other-than-temporary impairment of an investment in a private equity partnership pre-tax loss	0	0	$ 6,300,000
Net deferred debt issuance costs	36,300,000	36,700,000
Foreign subsidiary customer deposits	$ 273,400,000	$ 200,000,000
Average to include prudently incurred remediation costs	5 years
Property, Plant and Equipment
Estimated maximum period of capitalized and amortized costs to install partnership and antargaz-owned tanks	10 years
Estimated useful life of definite-lived intangible assets	15 years
Gas Utility
Property, Plant and Equipment
Depreciation expense as percentage of related average depreciable base	2.20%	2.30%	2.30%
Electric Utility
Property, Plant and Equipment
Depreciation expense as percentage of related average depreciable base	2.50%	2.50%	2.40%
Non-utility Plant and Equipment | Minimum
Property, Plant and Equipment
Useful life (in years)	10 years
Non-utility Plant and Equipment | Maximum
Property, Plant and Equipment
Useful life (in years)	40 years
Storage and Customer Tanks and Cylinders | Minimum
Property, Plant and Equipment
Useful life (in years)	6 years
Storage and Customer Tanks and Cylinders | Maximum
Property, Plant and Equipment
Useful life (in years)	40 years
Electricity Generation Facilities | Minimum
Property, Plant and Equipment
Useful life (in years)	25 years
Electricity Generation Facilities | Maximum
Property, Plant and Equipment
Useful life (in years)	40 years
Pipeline and Related Assets | Minimum
Property, Plant and Equipment
Useful life (in years)	25 years
Pipeline and Related Assets | Maximum
Property, Plant and Equipment
Useful life (in years)	40 years
Vehicles, Equipment and Office Furniture and Fixtures | Minimum
Property, Plant and Equipment
Useful life (in years)	3 years
Vehicles, Equipment and Office Furniture and Fixtures | Maximum
Property, Plant and Equipment
Useful life (in years)	12 years
Software and Software Development Costs [Member] | Maximum
Property, Plant and Equipment
Useful life (in years)	10 years
Other Assets
Property, Plant and Equipment
Cost method investments	$ 70,800,000	$ 77,800,000
Other Assets | Private Equity Partnership That Invests in Renewable Energy Companies
Property, Plant and Equipment
Cost method investments	$ 17,900,000	$ 17,400,000
Interest in a private equity partnership	3.50%