Summary of Significant Accounting Policies Materials, Supplies, and Inventories (Details) $ in Millions	12 Months Ended
Dec. 31, 2015 USD ($) $ / Dekatherm	Dec. 31, 2014 USD ($)
Inventory [Line Items]
Natural gas in storage	$ 284.1	$ 124.8
Materials and supplies	219.2	150.2
Fossil Fuel	183.7	125.6
Total	$ 687.0	$ 400.6
Percentage of LIFO Inventory	18.00%
PGL and NSG
Inventory [Line Items]
Excess of replacement or current costs over stated LIFO value	$ 15.2
Natural gas price benchmark \| $ / Dekatherm	2.48

Carrying amount as of the balance sheet date of merchandise, goods, commodities, or supplies relating to a source of usable power held for future sale or future use in manufacturing or production process.

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Carrying amount as of the balance sheet date of natural gas in storage, which is a mixture of gases (liquefied or otherwise), used for fuel and manufacturing purposes, which is ready for sale.

+ References

+ Details

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- Definition

Carrying amount as of the balance sheet date of other types of fuel derived from living matter of a previous geologic time, not otherwise itemized.

+ References

+ Details

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- Definition

The excess of replacement or current cost over the stated LIFO (last in first out) inventory value when the LIFO inventory method is utilized.

+ References

+ Details

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Aggregated amount of unprocessed materials to be used in manufacturing or production process and supplies that will be consumed. This amount is net of valuation reserves and adjustments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.6(a)(4),(5))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 330
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB TOPIC 5.BB)
-URI http://asc.fasb.org/extlink&oid=27011343&loc=d3e100047-122729

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 5
-Section BB

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The percentage of LIFO (last in first out) inventory to total inventory as of the balance sheet date if other than 100 percent.

+ References

No definition available.

+ Details

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- Definition

Represents the Chicago city-gate natural gas price used in calculating replacement amounts.

+ References

No definition available.

+ Details

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