XML 84 R62.htm IDEA: XBRL DOCUMENT v3.8.0.1
Regulatory and Rate Matters - TNMP Narrative (Details) - TNMP
 1 Months Ended 12 Months Ended
Jul. 30, 2011
USD ($)
Dec. 31, 2017
USD ($)
advanced_meter
Apr. 23, 2018
customer
Mar. 31, 2018
USD ($)
customer
Public Utilities, Commitments And Contingencies [Line Items]        
TCJA Regulatory liability for deferred income taxes   $ 146,500,000    
Regulatory liabilities       $ 1,500,000
Advanced Meter System Deployment and Surcharge Request        
Public Utilities, Commitments And Contingencies [Line Items]        
Approved deployment costs $ 113,400,000      
Collection of deployment costs through surcharge period 12 years      
Number of advanced meters installed (more than) | advanced_meter   242,000    
Recovery in cost through initial fees       200,000
Ongoing annual expenses       500,000
Approved non-standard metering ongoing expenses monthly charge       $ 36.78
Presumed number of customers that will elect non-standard meter service (in customers) | customer       1,081
Minimum | Advanced Meter System Deployment and Surcharge Request        
Public Utilities, Commitments And Contingencies [Line Items]        
Approved non-standard metering service cost, initial fee range       $ 63.97
Maximum | Advanced Meter System Deployment and Surcharge Request        
Public Utilities, Commitments And Contingencies [Line Items]        
Approved non-standard metering service cost, initial fee range       $ 168.61
Subsequent event | Advanced Meter System Deployment and Surcharge Request        
Public Utilities, Commitments And Contingencies [Line Items]        
Current number of customers that have elected non-standard meter service (in customers) | customer     98