Acquisitions - Narrative (Details) kWh in Millions	1 Months Ended		6 Months Ended	9 Months Ended	12 Months Ended
	Oct. 31, 2018 USD ($) kWh	Mar. 31, 2018 USD ($) usd_per_rce kWh market state	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($) kWh	Dec. 31, 2018 USD ($) kWh	Jun. 30, 2018 USD ($)
Business Acquisition [Line Items]
Balance in escrow account reflected as restricted cash			$ 1,001,000	$ 1,001,000	$ 8,636,000
Starion Energy
Business Acquisition [Line Items]
Total purchase price	$ 10,700,000
Residential customer equivalents (kwh) | kWh	600			510
Payments under purchase agreement to date				$ 8,000,000
Balance in escrow account reflected as restricted cash			1,000,000	$ 1,000,000	$ 8,600,000
Affiliated Entity | Asset Purchase Agreement
Business Acquisition [Line Items]
Residential customer equivalents (kwh) | kWh					350
Purchase price per RCE (in dollars per RCE) | usd_per_rce		250
Cash purchase price		$ 12,500,000.0
Paid to affiliate under the terms of the purchase agreement					$ 8,800,000
Net distribution to NG&E recorded in equity			$ 100,000		$ 7,100,000
Affiliated Entity | Asset Purchase Agreement | NG&E
Business Acquisition [Line Items]
Residential customer equivalents (kwh) | kWh		500			350
Number of markets | market		24
Number of states | state		8
Purchase price per RCE (in dollars per RCE) | usd_per_rce		250
Cash purchase price		$ 12,500,000
Paid to affiliate under the terms of the purchase agreement					$ 8,800,000
Transfer of assets						$ 1,700,000
Net distribution to NG&E recorded in equity					$ 7,100,000
HIKO
Business Acquisition [Line Items]
Total purchase price		$ 6,000,000
Residential customer equivalents (kwh) | kWh		290
Number of markets | market		42
Number of states | state		7