Summary of Accounting Policies Summary of Effect of Adoption on Consolidated Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Item Effected [Line Items]
Accrued revenue	$ 12,353		$ 12,353		$ 22,279
Regulated Energy Operating Revenue	70,504	$ 70,996	179,897	$ 168,650
Regulated Energy cost of sales	20,010	24,167	68,241	64,411
Depreciation and amortization	9,839	9,094	19,543	17,906
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	9,105	9,986	45,915	41,646
Income taxes	2,718	3,940	12,674	16,456
Net Income	6,387	$ 6,046	33,241	$ 25,190	58,124
Other assets	4,440		4,440		3,699
Retained earnings	250,377		250,377		$ 229,141
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Item Effected [Line Items]
Accrued revenue	13,659		13,659
Regulated Energy Operating Revenue	70,728		180,780
Regulated Energy cost of sales	20,139		68,942
Depreciation and amortization	9,832		19,521
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	9,207		46,119
Income taxes	2,746		12,733
Net Income	6,461		33,386
Other assets	4,777		4,777
Retained earnings	248,734		248,734
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Item Effected [Line Items]
Accrued revenue	(1,306)		(1,306)
Deferred Revenue, Period Increase (Decrease)	(224)		(883)
Regulated Energy cost of sales	(129)		(701)
Depreciation and amortization	7		22
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(102)		(204)
Income taxes	(28)		(59)
Net Income	(74)		(145)
Other assets	(337)		(337)
Retained earnings	$ 1,643		$ 1,643