Acquisitions - Components of Final Purchase Price Allocation (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2015	May 29, 2015
Liabilities assumed:
Goodwill	$ 2,989.0	$ 3,107.2	$ 2,953.4
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				117.5
Other noncurrent liabilities				113.4
Total liabilities assumed				443.6
Goodwill				183.8
Net consideration transferred (including working capital adjustments)				496.6
Intangible assets acquired, tradenames				12.0
Average amortization period	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