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Debt (Narrative) (Details) - USD ($) | 1 Months Ended | 6 Months Ended
--- | --- | ---
Jul. 31, 2019 | Jun. 30, 2019 | Apr. 30, 2019 | Jun. 30, 2019 | Jun. 30, 2018
Debt [Line Items] |  |  |  |  |
Credit facility agreement, extension |  |  |  | 1 year |
Line of credit facility, current borrowing capacity |  | $ 1,000,000,000 |  | $ 1,000,000,000 |
Payments to retire long-term debt |  |  |  | 3,400,000 | $ 503,000,000.0
IPL [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Line of credit facility, current borrowing capacity |  | 250,000,000 |  | 250,000,000 |
WPL [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Line of credit facility, current borrowing capacity |  | 300,000,000 |  | $ 300,000,000 |
Senior Debentures [Member] | 3.6% senior debentures due 2029 [Member] | IPL [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Proceeds from issuance of long-term debt |  |  | $ 300,000,000 |  |
Interest rate, percent |  |  | 3.60% |  |
Debentures [Member] | 3% debentures due 2029 [Member] | WPL [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Proceeds from issuance of long-term debt |  | $ 350,000,000 |  |  |
Interest rate, percent |  | 3.00% |  | 3.00% |
Debentures [Member] | 5% debentures due 2019 [Member] | WPL [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Interest rate, percent |  | 5.00% |  | 5.00% |
Parent Company [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Line of credit facility, current borrowing capacity |  | $ 450,000,000 |  | $ 450,000,000 |
Subsequent Event [Member] | Debentures [Member] | 5% debentures due 2019 [Member] | WPL [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Payments to retire long-term debt | $ 250,000,000 |  |  |  |

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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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