Summary of Significant Accounting Policies Fuel Inventories (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fuel inventories	$ 58.5us-gaap_PublicUtilitiesInventory	$ 74.4us-gaap_PublicUtilitiesInventory
Other Liabilities, Noncurrent	85.9us-gaap_OtherLiabilitiesNoncurrent	81.9us-gaap_OtherLiabilitiesNoncurrent
Public Utilities, Inventory, Fuel [Member]
Fuel inventories	66.7	74.4
Other Liabilities, Noncurrent	11.0
Other Assets	8.2
Enable Gas Transportation and Storage Agreement [Member] \| Public Utilities, Inventory, Fuel [Member]
Fuel inventories	$ 2.7

The aggregate carrying amounts, as of the balance sheet date, of assets not separately disclosed in the balance sheet.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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Aggregate carrying amount, as of the balance sheet date, of noncurrent obligations not separately disclosed in the balance sheet. Noncurrent liabilities are expected to be paid after one year (or the normal operating cycle, if longer).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Subparagraph (SX 210.5-02.24)

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The period end amount for a type of inventory held by the utility in a schedule of inventories.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Inventory

-URI http://asc.fasb.org/extlink&oid=6516093

Reference 2: http://www.xbrl.org/2003/role/presentationRef

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