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Long-Term Debt (Narrative) (Details) - USD ($) $ in Millions |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jul. 16, 2018 | Sep. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jun. 11, 2018 | Mar. 15, 2018 | Nov. 17, 2017 | Sep. 15, 2017 | Sep. 14, 2017 | Aug. 04, 2017 | Jul. 01, 2017 | Jun. 12, 2017 | May 22, 2017 | Apr. 03, 2017 | Mar. 27, 2017
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Convertible, Conversion Ratio |  |  |  |  | 61.4 |  |  |  |  |  |  |  |  |  |  |  |  |
Face amount of notes |  |  |  |  |  |  |  |  |  | $ 500.0 |  |  |  |  |  |  |  |
Debt Instrument Tendered |  |  |  |  | $ 209.0 | $ 990.7 |  |  |  |  |  |  |  |  |  |  |  |
Payment for Debt Extinguishment or Debt Prepayment Cost |  |  |  |  | 45.5 | 111.5 | $ 0.0 |  |  |  |  |  |  |  |  |  |  |
Loss on early extinguishment of long-term debt |  | $ 33.0 | $ 111.5 | $ 12.5 | (45.5) | $ (111.5) | $ 0.0 |  |  |  |  |  |  |  |  |  |  |
Cross default provision on default relating to any indebtedness |  |  |  |  | 50.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Security interest and other subset of asset |  |  |  |  | $ 150.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Asset sale covenant percentage of consolidated total assets |  |  |  |  | 10.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of additional subset of assets capped |  |  |  |  | 10.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Face amount of notes |  |  |  |  | $ 2,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cross default provision on default relating to any indebtedness |  |  |  |  | 50.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cross default provision on default relating to any indebtedness |  |  |  |  | 5.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Interest Rate Swap |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  |  |  |  | $ 500.0 |  |  |  |  |  |  |  |  |  |  | $ 950.0 |  |
Treasury Lock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  |  |  |  |  |  |  |  |  | 250.0 |  | $ 750.0 |  |  |  |  |  |
Senior Note Redeemed 2018 Note 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on early extinguishment of long-term debt | $ 33.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
3.65% Notes Due 2023 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate on debt |  |  |  |  | 3.65% |  |  |  |  |  |  |  |  |  |  |  |  |
3.65% Notes Due 2023 | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Face amount of notes |  |  |  |  |  |  |  | $ 350.0 |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance of Debt |  |  |  |  | $ 346.6 |  |  |  |  |  |  |  |  |  |  |  |  |
6.80% Notes Due 2019 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount |  |  |  |  | 551.1 |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate on debt |  |  |  |  |  | 6.80% |  |  |  |  |  |  |  |  |  |  |  |
Medium Term Note Redeemed 2017 Note 9 | Medium-term Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount |  |  |  |  |  |  |  |  |  |  | $ 210.4 |  |  |  |  |  |  |
Medium Term Note Redeemed 2017 Note 9 | NIPSCO | Medium-term Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 5.0 |  |  |  |  |
Debt, Weighted Average Interest Rate |  |  |  |  |  |  |  |  |  |  | 5.25% |  | 7.02% |  |  |  |  |
3.95% Notes Due 2048 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate on debt |  |  |  |  |  |  |  |  |  |  |  | 3.95% |  |  |  |  |  |
3.95% Notes Due 2048 | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Face amount of notes |  |  |  |  |  |  |  |  |  |  |  | $ 750.0 |  |  |  |  |  |
Medium Term Note Redeemed 2017 Note 7 | NIPSCO | Medium-term Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 22.5 |  |  |
Debt, Weighted Average Interest Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.59% |  |  |
5.45% Notes due 2020 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate on debt |  |  |  |  |  | 5.45% |  |  |  |  |  |  |  |  |  |  |  |
6.40% Notes Due 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount |  |  |  |  |  |  |  |  | $ 275.1 |  |  |  |  |  |  |  |  |
Interest rate on debt |  |  |  |  |  | 6.40% |  |  | 6.40% |  |  |  |  |  |  |  |  |
4.375% Notes due 2047 | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Face amount of notes |  |  |  |  | $ 1,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate on debt |  |  |  |  | 4.375% |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes Redeemed 2018 Note 1 | Medium-term Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 30.0
Interest rate on debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.86%
Medium Term Note Redeemed 2017 Note 2 | Medium-term Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2.0
Interest rate on debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.85%
Medium Term Note Redeemed 2017 Note 3 | Medium-term Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 12.0 |
Interest rate on debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.82% |
Medium Term Note Redeemed 2017 Note 4 | Medium-term Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 10.0 |
Interest rate on debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.92% |
Medium Term Note Redeemed 2017 Note 5 | Medium-term Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2.0 |
Interest rate on debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.93% |
Medium Term Note Redeemed 2017 Note 6 | Medium-term Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1.0 |
Interest rate on debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7.94% |
6.13% Notes Due 2022 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate on debt |  |  |  |  |  | 6.125% |  |  |  |  |  |  |  |  |  |  |  |
Medium Term Note Redeemed 2017 Note 8 | NIPSCO | Medium-term Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 55.0 |  |  |  |
Debt, Weighted Average Interest Rate |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.70% |  |  |  |
3.49% Notes due 2027 | Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Face amount of notes |  |  |  |  | $ 1,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate on debt |  |  |  |  | 3.49% |  |  |  |  |  |  |  |  |  |  |  |  |
2.65% Notes Due 2022 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Face amount of notes |  |  |  |  |  |  |  |  |  | $ 500.0 |  |  |  |  |  |  |  |
Interest rate on debt |  |  |  |  |  |  |  |  |  | 2.65% |  |  |  |  |  |  |  |
Revolving Credit Facility | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Convertible, Conversion Ratio |  |  |  |  | 70 |  |  |  |  |  |  |  |  |  |  |  |  |
Private Placement [Member] | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Convertible, Conversion Ratio |  |  |  |  | 75 |  |  |  |  |  |  |  |  |  |  |  |  |

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