Restatements of Condensed Consolidated Financial Statements - Condensed Consolidated Statements of Cash Flows (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Net income	$ 605.3	$ 509.5
Depreciation and amortization	181.7	177.4
Deferred income taxes, net	13.0	19.4
Net change in realized gains and losses deferred as cash flow hedges		0
Unrealized gains on derivative instruments	8.0	(5.8)
Other, net	(6.8)	11.5
Accounts receivable and accrued utility revenues	(632.5)	(555.2)
Inventories	43.0	61.7
Accounts payable	194.1	176.4
As Previously Reported
Net income		496.9
Depreciation and amortization		174.4
Deferred income taxes, net		18.8
Net change in realized gains and losses deferred as cash flow hedges		8.3
Unrealized gains on derivative instruments		0
Other, net		7.1
Accounts receivable and accrued utility revenues		(550.1)
Inventories		64.5
Accounts payable		175.0
Adjustment
Net income		12.6
Depreciation and amortization		3.0
Deferred income taxes, net		0.6
Net change in realized gains and losses deferred as cash flow hedges		(8.3)
Unrealized gains on derivative instruments		(5.8)
Other, net		4.4
Accounts receivable and accrued utility revenues		(5.1)
Inventories		(2.8)
Accounts payable		$ 1.4