Significant Segment Financial Data (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	$ 104,300	$ 78,800	$ 74,500	$ 125,800	$ 102,400	$ 74,700	$ 77,500	$ 172,200	$ 383,400	$ 426,800	$ 425,800
Interest Income									1,200	1,900	1,300
Interest Expense									23,700	23,800	22,200
Depreciation & Amortization Expense									46,600	45,700	42,100
Income Tax Expense (Benefit)									15,354	15,443	13,995
Segment Profit	10,200	$ 3,500	$ 2,500	$ 10,900	9,300	$ 1,700	$ 1,700	$ 13,600	27,100	26,300	24,700
Segment Assets	1,128,200				1,038,800				1,128,200	1,038,800	997,000
Capital Expenditures									98,100	103,900	92,600
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									181,200	202,600	201,400
Interest Income									200	800	300
Interest Expense									13,300	13,300	11,500
Depreciation & Amortization Expense									21,900	20,700	18,800
Income Tax Expense (Benefit)									9,200	10,200	10,800
Segment Profit									14,500	15,300	15,800
Segment Assets	645,200				590,900				645,200	590,900	564,900
Capital Expenditures									57,000	64,900	62,300
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									196,100	218,000	218,700
Interest Income									700	700	600
Interest Expense									8,300	8,800	9,100
Depreciation & Amortization Expense									23,800	24,000	22,300
Income Tax Expense (Benefit)									6,600	5,500	4,500
Segment Profit									11,100	8,700	6,800
Segment Assets	441,100				415,100				441,100	415,100	412,600
Capital Expenditures									30,100	29,900	24,800
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest Income									200	300	300
Interest Expense									2,100	1,700	1,600
Depreciation & Amortization Expense									800	900	1,000
Income Tax Expense (Benefit)									(1,200)	(1,100)	(1,900)
Segment Profit									400	1,000	1,200
Segment Assets	35,100				26,200				35,100	26,200	13,200
Capital Expenditures									11,000	9,000	5,200
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									6,100	6,200	5,700
Interest Income									100	100	100
Depreciation & Amortization Expense									100	100
Income Tax Expense (Benefit)									800	800	600
Segment Profit									1,100	1,300	900
Segment Assets	$ 6,800				$ 6,600				$ 6,800	6,600	6,300
Capital Expenditures										$ 100	$ 300