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Regulatory and Rate Matters - TNMP Narrative (Details) $ in Millions			1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Nov. 02, 2018 USD ($)	May 30, 2018 USD ($)	Jul. 30, 2011 USD ($)	Sep. 30, 2018 USD ($) advanced_meter	Sep. 30, 2018 USD ($) advanced_meter	Dec. 31, 2017 USD ($)	Aug. 06, 2018 USD ($)
Public Utilities, Commitments And Contingencies [Line Items]
Unrecovered investment revenue		$ 21.8
TNMP
Public Utilities, Commitments And Contingencies [Line Items]
Net regulatory liability						$ 37.8
Reduction to revenue				$ 1.5	$ 4.2
Energy efficiency cost recovery, requested change amount		5.7					$ 5.6
Energy efficiency cost recovery, requested bonus		0.9		$ 0.8	$ 0.8		$ 0.8
TNMP | 2018 TNMP Rate Case
Public Utilities, Commitments And Contingencies [Line Items]
Requested rate increase (decrease)		$ 25.9
Requested return on equity		10.50%
Requested cost of debt		7.20%
Requested debt capital structure		50.00%
Requested equity capital structure		50.00%
New rate rider recovery, amount		$ 7.7
Regulatory liabilities						146.5
Net regulatory liability						$ 14.4
TNMP | Advanced Meter System Deployment and Surcharge Request
Public Utilities, Commitments And Contingencies [Line Items]
Approved deployment costs			$ 113.4
Collection of deployment costs through surcharge period			12 years
Number of advanced meters installed (more than) | advanced_meter				242,000	242,000
Subsequent event | TNMP | 2018 TNMP Rate Case
Public Utilities, Commitments And Contingencies [Line Items]
Requested rate increase (decrease)	$ 10.0
Requested return on equity	9.65%
Requested cost of debt	6.44%
Requested debt capital structure	55.00%
Requested equity capital structure	45.00%
Investments excluded from rate request	$ 11.7

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

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