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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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