Significant Segment Financial Data (Detail) - 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
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue - Non-Regulated									$ 4,700
Revenues	$ 125,600	$ 88,200	$ 84,500	$ 145,800	$ 115,400	$ 84,000	$ 80,800	$ 126,000	444,100	$ 406,200	$ 383,400
Interest Income									2,400	2,400	1,200
Interest Expense									26,400	25,500	23,700
Depreciation & Amortization Expense									50,400	46,900	46,600
Income Tax Expense (Benefit)									8,393	17,537	15,354
Segment Profit	11,000	$ 2,800	$ 3,600	$ 15,600	11,200	$ 2,300	$ 3,100	$ 12,400	33,000	29,000	27,100
Segment Assets	1,298,300				1,241,900				1,298,300	1,241,900	1,128,200
Capital Expenditures									102,400	119,300	98,100
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									439,400
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									216,100	194,000	181,200
Interest Income									800	700	200
Interest Expense									14,200	13,700	13,300
Depreciation & Amortization Expense									24,900	22,400	21,900
Income Tax Expense (Benefit)									7,100	10,700	9,200
Segment Profit									18,800	16,400	14,500
Segment Assets	764,100				714,300				764,100	714,300	645,200
Capital Expenditures									70,800	72,100	57,000
Gas Segment | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									210,700
Gas Segment | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									5,400
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									223,300	206,200	196,100
Interest Income									800	1,000	700
Interest Expense									9,000	8,800	8,300
Depreciation & Amortization Expense									23,100	23,400	23,800
Income Tax Expense (Benefit)									4,200	7,500	6,600
Segment Profit									11,400	11,900	11,100
Segment Assets	484,200				476,900				484,200	476,900	441,100
Capital Expenditures									28,400	33,700	30,100
Electric | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									228,700
Electric | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									(5,400)
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest Income									600	600	200
Interest Expense									3,200	3,000	2,100
Depreciation & Amortization Expense									2,300	1,000	800
Income Tax Expense (Benefit)									(3,400)	(1,400)	(1,200)
Segment Profit									1,500	(500)	400
Segment Assets	43,100				44,000				43,100	44,000	35,100
Capital Expenditures									3,200	13,500	11,000
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue - Non-Regulated									4,700
Revenues									4,700	6,000	6,100
Interest Income									200	100	100
Depreciation & Amortization Expense									100	100	100
Income Tax Expense (Benefit)									500	700	800
Segment Profit									1,300	1,200	1,100
Segment Assets	$ 6,900				$ 6,700				$ 6,900	$ 6,700	$ 6,800