Summary of Accounting Policies Summary of Effect of Adoption on Consolidated Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Item Effected [Line Items]
Accrued revenue	$ 18,907		$ 22,279
Regulated Energy Operating Revenue	109,393	$ 97,654
Regulated Energy cost of sales	48,231	40,244
Depreciation and amortization	9,704	8,812
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	36,810	31,660
Income taxes	9,955	12,516
Net Income	26,855	$ 19,144	58,124
Other assets	4,316		3,699
Retained earnings	250,024		$ 229,141
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Item Effected [Line Items]
Accrued revenue	20,213
Regulated Energy Operating Revenue	110,357
Regulated Energy cost of sales	48,803
Depreciation and amortization	9,689
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	37,217
Income taxes	10,077
Net Income	27,140
Other assets	4,508
Retained earnings	251,522
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Item Effected [Line Items]
Accrued revenue	(1,306)
Deferred Revenue, Period Increase (Decrease)	(964)
Regulated Energy cost of sales	(572)
Depreciation and amortization	15
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(407)
Income taxes	(122)
Net Income	(285)
Other assets	(192)
Retained earnings	$ (1,498)