Summary of Accounting Policies Acquisitions Additional information (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Revenues	$ 130,903	$ 136,664	$ 358,519	$ 376,020
Operating Income (Loss)	17,436	13,248	61,429	53,654
Marlin and Ohl [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred					$ 18,400
Marlin [Member]
Business Acquisition [Line Items]
Revenues	1,108		3,541
Operating Income (Loss)	48		1,423
Ohl [Member]
Business Acquisition [Line Items]
Revenues	174		997
Operating Income (Loss)	(61)		212
Public Utilities, Property, Plant and Equipment, Amount of Acquisition Adjustments	200		200
Unregulated Energy [Member]
Business Acquisition [Line Items]
Operating Income (Loss)	$ (1,348)	$ 490	$ 13,628	$ 14,174
Unregulated Energy [Member] | Marlin [Member]
Business Acquisition [Line Items]
Goodwill, Acquired During Period					4,800
Unregulated Energy [Member] | Ohl [Member]
Business Acquisition [Line Items]
Goodwill, Acquired During Period					$ 1,500
Number of customers acquired through acquisition					2,500