Regulatory and Rate Matters - TNMP Narrative (Details) - USD ($) $ in Millions			3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 20, 2018	May 30, 2018	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Mar. 01, 2019
Public Utilities, Commitments And Contingencies [Line Items]
Unrecovered investment revenue		$ 20.2
TNMP
Public Utilities, Commitments And Contingencies [Line Items]
Net regulatory liability						$ 37.8
Reduction to revenue			$ 1.2	$ 2.7	$ 5.4
Energy efficiency cost recovery, requested change amount		5.9					$ 5.6
Energy efficiency cost recovery, requested bonus		0.8					$ 0.8
TNMP | 2018 TNMP Rate Case
Public Utilities, Commitments And Contingencies [Line Items]
Requested rate increase (decrease)	$ 10.0	$ 25.9
Requested return on equity	9.65%	10.50%
Requested cost of debt	6.44%	7.20%
Requested debt capital structure	55.00%	50.00%
Requested equity capital structure	45.00%	50.00%
Investments excluded from rate request	$ 10.6
Refund of federal income tax rates period						5 years