Commitments and Contingencies (Details) $ in Millions			6 Months Ended
	Jun. 07, 2019 USD ($)	May 03, 2019	Jun. 30, 2019 USD ($) investigation	Dec. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Number of current investigations | investigation			2
Failure to Comply With Regulations, Notice of Violation and Assessment of Civil Penalty
Loss Contingencies [Line Items]
Notice of violation and assessment of civil penalty			$ 0.9
Failure to Comply With Regulations, Notice of Violation of Marketing Practices and Assessment of Civil Penalty
Loss Contingencies [Line Items]
Notice of violation and assessment of civil penalty			0.8
Litigation and Regulatory Matters
Loss Contingencies [Line Items]
Contingent liabilities			18.7	$ 0.9
Indirect Tax Audits
Loss Contingencies [Line Items]
Contingent liabilities			$ 0.4	$ 0.6
Major Energy Companies v. National Gas & Electric, LLC (NG&E) and Spark Energy, Inc. (Spark) | Pending Litigation
Loss Contingencies [Line Items]
Plaintiff’s payment of fees and costs (less than)	$ 0.1
Verde Ohio | Investigation by the Public Utility Commission of Ohio | Pending Litigation
Loss Contingencies [Line Items]
Term of settlement agreement		30 days