Acquisitions - Allocation of Purchase Price (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Liabilities assumed:
Goodwill	$ 2,953.4	$ 2,833.4	$ 2,873.7
Totalgaz SAS
Assets acquired:
Cash	86.8
Accounts receivable	170.3
Prepaid expenses and other current assets	11.0
Property, plant & equipment	375.6
Intangible assets	91.3
Other assets	21.4
Total assets acquired	756.4
Liabilities assumed:
Accounts payable	109.2
Other current liabilities	103.5
Deferred income taxes	115.8
Other noncurrent liabilities	(117.5)
Total liabilities assumed	(446.0)
Goodwill	186.2
Net consideration transferred (including working capital adjustments)	496.6
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Intangible Assets, Tradenames	$ 12.0
Average amortization period (in years)	15 years
Totalgaz SAS | Customer Relationships
Liabilities assumed:
Finite-lived intangible assets acquired	$ 79.3
Totalgaz SAS | Tradenames
Liabilities assumed:
Finite-lived intangible assets acquired	$ 8.3