Summary of Significant Accounting Policies Effects of New Accounting Pronouncements (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Regulated Energy									$ 345,281	$ 326,310	$ 305,689
Accrued revenue	$ 27,499				$ 22,279				27,499	22,279
Receivables and other deferred charges	6,985				3,699				6,985	3,699
Retained earnings	261,530				229,141				261,530	229,141
Regulated Energy cost of sales									121,828	118,769	109,609
Income Tax Expense (Benefit)									20,994	14,309	28,341
Depreciation and amortization									40,802	36,599	32,159
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									77,574	72,433	73,016
Net Income	17,801	$ 5,538	$ 6,387	$ 26,855	$ 26,101	$ 6,833	$ 6,046	$ 19,144	56,580	$ 58,124	$ 44,675
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Regulated Energy									346,289
Accrued revenue	29,461								29,461
Receivables and other deferred charges	6,816								6,816
Retained earnings	263,323								263,323
Regulated Energy cost of sales									122,463
Income Tax Expense (Benefit)									21,106
Depreciation and amortization									40,767
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									77,981
Net Income									56,875
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accrued revenue	(1,962)								(1,962)
Receivables and other deferred charges	169								169
Retained earnings	$ (1,793)								(1,793)
Deferred Revenue, Period Increase (Decrease)									(1,008)
Regulated Energy cost of sales									(635)
Income Tax Expense (Benefit)									(112)
Depreciation and amortization									35
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									(407)
Net Income									$ (295)