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Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Commodity Contracts [Member] |  |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Beginning balance |  | $ 9.2 | $ 0.6 | $ 8.7 | $ (32.7)
Total net gains (losses) included in changes in net assets (realized/unrealized) |  | (4.3) | (5.1) | (31.3) | 8.0
Transfers into Level 3 |  |  |  | 0.0 | 0.9
Transfers out of Level 3 |  | 0.0 | 0.8 | 12.2 | 1.2
Purchases |  |  |  | 28.3 | 22.0
Sales |  | (0.1) | (0.2) | (0.3) | (0.9)
Settlements |  | (8.5) | (4.0) | (21.3) | (6.4)
Ending balance |  | (3.7) | (7.9) | (3.7) | (7.9)
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized (gains) losses relating to assets and liabilities held at September 30 |  | (4.2) | (5.0) | (29.4) | 9.7
Commodity Contracts [Member] | IPL [Member] |  |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Beginning balance |  | 17.1 | 18.3 | 10.1 | (1.9)
Total net gains (losses) included in changes in net assets (realized/unrealized) |  | (4.4) | (0.4) | (13.9) | 4.8
Transfers into Level 3 |  |  |  | 0.0 | 0.5
Transfers out of Level 3 |  | 0.0 | 0.3 | 3.1 | 0.2
Purchases |  |  |  | 24.6 | 20.6
Sales |  | (0.1) | (0.2) | (0.2) | (0.9)
Settlements |  | (7.3) | (4.6) | (18.4) | (9.9)
Ending balance |  | 5.3 | 13.4 | 5.3 | 13.4
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized (gains) losses relating to assets and liabilities held at September 30 |  | (4.5) | (0.4) | (12.6) | 5.7
Commodity Contracts [Member] | WPL [Member] |  |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Beginning balance |  | (7.9) | (17.7) | (1.4) | (30.8)
Total net gains (losses) included in changes in net assets (realized/unrealized) |  | 0.1 | (4.7) | (17.4) | 3.2
Transfers into Level 3 |  |  |  | 0.0 | 0.4
Transfers out of Level 3 |  | 0.0 | 0.5 | 9.1 | 1.0
Purchases |  |  |  | 3.7 | 1.4
Sales |  |  |  | (0.1) | 0.0
Settlements |  | (1.2) | (0.6) | (2.9) | (3.5)
Ending balance |  | (9.0) | (21.3) | (9.0) | (21.3)
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized (gains) losses relating to assets and liabilities held at September 30 |  | 0.3 | (4.6) | (16.8) | 4.0
Deferred Proceeds [Member] |  |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Beginning balance |  | 170.0 | 74.4 | 211.1 | 172.0
Total net gains (losses) included in changes in net assets (realized/unrealized) |  | 0.0 | 0.0 | 0.0 | 0.0
Transfers into Level 3 |  |  |  | 0.0 | 0.0
Transfers out of Level 3 |  | 0.0 | 0.0 | 0.0 | 0.0
Purchases |  |  |  | 0.0 | 0.0
Sales |  | 0.0 | 0.0 | 0.0 | 0.0
Settlements | [1] | (54.7) | 165.3 | (95.8) | 67.7
Ending balance |  | 115.3 | 239.7 | 115.3 | 239.7
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized (gains) losses relating to assets and liabilities held at September 30 |  | 0.0 | 0.0 | 0.0 | 0.0
Deferred Proceeds [Member] | IPL [Member] |  |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Beginning balance |  | 170.0 | 74.4 | 211.1 | 172.0
Total net gains (losses) included in changes in net assets (realized/unrealized) |  | 0.0 | 0.0 | 0.0 | 0.0
Transfers into Level 3 |  |  |  | 0.0 | 0.0
Transfers out of Level 3 |  | 0.0 | 0.0 | 0.0 | 0.0
Purchases |  |  |  | 0.0 | 0.0
Sales |  | 0.0 | 0.0 | 0.0 | 0.0
Settlements | [1] | (54.7) | 165.3 | (95.8) | 67.7
Ending balance |  | 115.3 | 239.7 | 115.3 | 239.7
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized (gains) losses relating to assets and liabilities held at September 30 |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0
[1] | Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold.

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- Definition Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Amount of transfers of financial instrument classified as a liability into level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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