Regulatory and Rate Matters - Transmission Cost of Service Rates (Details) - TNMP - USD ($) $ in Millions	Apr. 06, 2020	Mar. 27, 2020	Sep. 19, 2019	Mar. 21, 2019
Subsequent event
Public Utilities, General Disclosures [Line Items]
Requested increase in annual distribution revenue	$ 14.7
Incremental distribution investments	$ 149.2
PUCT
Public Utilities, General Disclosures [Line Items]
Approved Increase in Rate Base		$ 59.2	$ 21.9	$ 111.8
Annual Increase in Revenue		$ 7.8	$ 3.3	$ 14.3