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ALAGASCO ACQUISITION - Alabama Gas Corporation (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended		1 Months Ended
	Sep. 02, 2014	Jan. 06, 2015	Jan. 31, 2015	Dec. 31, 2014	Sep. 30, 2014
Business Acquisition [Line Items]
Goodwill				$ 937.8us-gaap_Goodwill	$ 937.8us-gaap_Goodwill
Alagasco
Business Acquisition [Line Items]
Percentage of voting interest acquired	100.00%
Total consideration transferred	1,600.0
Payments to acquire businesses, net of cash acquired	1,305.2
Goodwill				727.6	727.6
Subsequent Event \| Alagasco
Business Acquisition [Line Items]
Payments to acquire businesses, net of cash acquired			1,313.8
Final reconciliation of net assets payments to acquire businesses		8.6
Goodwill			$ 736.2