Significant Segment Financial Data (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue – Non-Regulated										$ 900	$ 4,700
Total Operating Revenues	$ 116,900	$ 87,400	$ 83,900	$ 130,400	$ 116,400	$ 85,300	$ 84,400	$ 152,100	$ 418,600	438,200	444,100
Interest Income									2,600	2,900	2,400
Interest Expense									26,400	26,600	26,400
Depreciation & Amortization Expense									54,500	52,000	50,400
Income Tax Expense (Benefit)									10,249	13,808	8,393
Segment Profit	13,600	$ 300	$ 3,100	$ 15,200	11,400	$ 2,300	$ 4,000	$ 26,500	32,200	44,200	33,000
Segment Assets	1,477,900				1,370,800				1,477,900	1,370,800	1,298,300
Capital Expenditures									122,600	119,200	102,400
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues									411,900	435,200	439,400
Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues									6,700	2,100
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues									191,400	203,400	216,100
Interest Income									1,100	1,200	800
Interest Expense									14,200	14,400	14,200
Depreciation & Amortization Expense									29,800	28,500	24,900
Income Tax Expense (Benefit)									7,300	7,200	7,100
Segment Profit									19,300	19,100	18,800
Segment Assets	886,300				823,300				886,300	823,300	764,100
Capital Expenditures									71,100	74,000	70,800
Gas Segment | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues									185,200	212,100	210,700
Gas Segment | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues									6,200	(8,700)	5,400
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues									227,200	233,900	223,300
Interest Income									1,100	900	800
Interest Expense									8,700	9,400	9,000
Depreciation & Amortization Expense									23,800	22,600	23,100
Income Tax Expense (Benefit)									4,700	4,200	4,200
Segment Profit									12,900	11,500	11,400
Segment Assets	571,800				529,300				571,800	529,300	484,200
Capital Expenditures									45,500	39,600	28,400
Electric | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues									226,700	223,100	228,700
Electric | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues									500	10,800	(5,400)
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest Income									400	600	600
Interest Expense									3,500	2,800	3,200
Depreciation & Amortization Expense									900	900	2,300
Income Tax Expense (Benefit)									(1,800)	(1,400)	(3,400)
Segment Profit									0	3,400	1,500
Segment Assets	$ 19,800				17,900				19,800	17,900	43,100
Capital Expenditures									$ 6,000	5,600	3,200
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue – Non-Regulated										900	4,700
Total Operating Revenues										900	4,700
Interest Income										200	200
Depreciation & Amortization Expense											100
Income Tax Expense (Benefit)										3,800	500
Segment Profit										10,200	1,300
Segment Assets					$ 300					$ 300	$ 6,900