Transactions with Affiliates (Details) $ / shares in Units, kWh in Millions		1 Months Ended	3 Months Ended		6 Months Ended		9 Months Ended	12 Months Ended
	Jun. 13, 2019 USD ($)	Mar. 31, 2018 USD ($) usd_per_rce kWh	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) kWh
Related Party Transaction [Line Items]
Net amount direct billed and allocated from affiliates			$ 37,247,000	$ 27,780,000	$ 66,723,000	$ 57,827,000
Current accounts receivable - affiliates			3,882,000		3,882,000		$ 2,558,000	$ 2,558,000
Current accounts payable - affiliates			2,475,000		2,475,000		2,464,000	2,464,000
Revenues recorded in net asset optimization revenues related to affiliates			(56,000)	763,000	2,496,000	3,450,000
Cash received for disgorgement of stockholders short-swing profits (less than for the three months ended June 30, 2019)			100,000		$ 55,000	244,000
Tax receivable agreement, net cash savings (as a percent)					15.00%
TRA payment					$ 0	3,577,000	6,200,000
Liability related to TRA, current			11,239,000		11,239,000		1,658,000	1,658,000
Affiliated Entity | Subordinated Debt Facility
Related Party Transaction [Line Items]
Subordinated debt—affiliate	$ 25,000,000.0
Subordinated debt, advances	$ 1,000,000.0
Subordinated debt, interest rate on advances	5.00%
Outstanding under subordinated note			0		0		10,000,000	$ 10,000,000
Affiliated Entity | Allocated Overhead Costs
Related Party Transaction [Line Items]
Net amount direct billed and allocated from affiliates			400,000	1,500,000	400,000	8,400,000
Affiliated Entity | Purchased Natural Gas From Affiliate
Related Party Transaction [Line Items]
Cost of revenues recorded in net asset optimization revenues (less than)			100,000	100,000	100,000	100,000
Affiliated Entity | Asset Purchase Agreement
Related Party Transaction [Line Items]
Residential customer equivalents (kwh) | kWh								350
Purchase price per RCE (in dollars per RCE) | usd_per_rce		250
Cash purchase price (up to)		$ 12,500,000.0
Paid to affiliate under the terms of the purchase agreement							8,800,000	$ 8,800,000
Net distribution to NG&E recorded in equity (less than as of June 30, 2019)					100,000			$ 7,100,000
NuDevco Retail Holdings and NuDevco Retail
Related Party Transaction [Line Items]
Distributions to affiliates					$ 7,500,000	7,800,000
Tax receivable agreement, net cash savings (as a percent)					85.00%
Tax receivable agreement, deferral period					5 years
Tax receivable agreement, coverage percentage					130.00%
NuDevco Retail Holdings and NuDevco Retail | Common Class A
Related Party Transaction [Line Items]
Tax receivable agreement, target dividend (in dollars per share) | $ / shares					$ 0.18125
NuDevco Retail Holdings and NuDevco Retail | Series A Preferred Stock
Related Party Transaction [Line Items]
Tax receivable agreement, target dividend (in dollars per share) | $ / shares					$ 0.546875
NuDevco Retail Holdings and NuDevco Retail | Payment of Income Taxes Incurred by the Company
Related Party Transaction [Line Items]
Distributions to affiliates					$ 0	11,700,000
National Gas & Electric, LLC | Affiliated Entity | Asset Purchase Agreement
Related Party Transaction [Line Items]
Residential customer equivalents (kwh) | kWh		500						350
Purchase price per RCE (in dollars per RCE) | usd_per_rce		250
Cash purchase price (up to)		$ 12,500,000
Paid to affiliate under the terms of the purchase agreement							8,800,000	$ 8,800,000
Net distribution to NG&E recorded in equity (less than as of June 30, 2019)								7,100,000
Retailco | Affiliated Entity | Allocated Overhead Costs
Related Party Transaction [Line Items]
Net amount direct billed and allocated from affiliates			0	500,000	0	5,800,000
Retailco | Affiliated Entity | Property and Equipment Capitalized
Related Party Transaction [Line Items]
Capitalized under Master Service Agreement (less than for three months ended June 30, 2018)			0	100,000	0	500,000
Retailco and NuDevco Retail | Tax Receivable Agreement
Related Party Transaction [Line Items]
Total liability related to TRA			27,600,000		27,600,000		27,600,000	27,600,000
Liability related to TRA, current			11,200,000		11,200,000		$ 1,700,000	$ 1,700,000
Affiliated Entity
Related Party Transaction [Line Items]
Revenues recorded in net asset optimization revenues related to affiliates			$ 600,000	$ 400,000	$ 1,800,000	$ 1,000,000