Significant Segment Financial Data (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	$ 115,400	$ 84,000	$ 80,800	$ 126,000	$ 104,300	$ 78,800	$ 74,500	$ 125,800	$ 406,200	$ 383,400	$ 426,800
Interest Income									2,400	1,200	1,900
Interest Expense									25,500	23,700	23,800
Depreciation & Amortization Expense									46,900	46,600	45,700
Income Tax Expense (Benefit)									17,537	15,354	15,443
Segment Profit	11,200	$ 2,300	$ 3,100	$ 12,400	10,200	$ 3,500	$ 2,500	$ 10,900	29,000	27,100	26,300
Segment Assets	1,241,900				1,128,200				1,241,900	1,128,200	1,038,800
Capital Expenditures									119,300	98,100	103,900
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									194,000	181,200	202,600
Interest Income									700	200	800
Interest Expense									13,700	13,300	13,300
Depreciation & Amortization Expense									22,400	21,900	20,700
Income Tax Expense (Benefit)									10,700	9,200	10,200
Segment Profit									16,400	14,500	15,300
Segment Assets	714,300				645,200				714,300	645,200	590,900
Capital Expenditures									72,100	57,000	64,900
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									206,200	196,100	218,000
Interest Income									1,000	700	700
Interest Expense									8,800	8,300	8,800
Depreciation & Amortization Expense									23,400	23,800	24,000
Income Tax Expense (Benefit)									7,500	6,600	5,500
Segment Profit									11,900	11,100	8,700
Segment Assets	476,900				441,100				476,900	441,100	415,100
Capital Expenditures									33,700	30,100	29,900
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest Income									600	200	300
Interest Expense									3,000	2,100	1,700
Depreciation & Amortization Expense									1,000	800	900
Income Tax Expense (Benefit)									(1,400)	(1,200)	(1,100)
Segment Profit									(500)	400	1,000
Segment Assets	44,000				35,100				44,000	35,100	26,200
Capital Expenditures									13,500	11,000	9,000
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									6,000	6,100	6,200
Interest Income									100	100	100
Depreciation & Amortization Expense									100	100	100
Income Tax Expense (Benefit)									700	800	800
Segment Profit									1,200	1,100	1,300
Segment Assets	$ 6,700				$ 6,800				$ 6,700	$ 6,800	6,600
Capital Expenditures											$ 100