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Significant Segment Financial Data (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	$ 102,400	$ 74,700	$ 77,500	$ 172,200	$ 119,800	$ 76,600	$ 73,300	$ 156,100	$ 426,800	$ 425,800	$ 366,900
Interest Income									1,900	1,300	3,200
Interest Expense									23,800	22,200	22,000
Depreciation & Amortization Expense									45,700	42,100	38,500
Income Tax Expense (Benefit)									15,443	13,995	12,652
Segment Profit	9,300	$ 1,700	$ 1,700	$ 13,600	9,400	$ 1,600	$ 1,100	$ 12,600	26,300	24,700	21,600
Segment Assets	1,046,400				1,000,200				1,046,400	1,000,200	920,600
Capital Expenditures									103,900	92,600	89,500
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									202,600	201,400	170,400
Interest Income									800	300	500
Interest Expense									13,300	11,500	11,000
Depreciation & Amortization Expense									20,700	18,800	17,200
Income Tax Expense (Benefit)									10,200	10,800	7,500
Segment Profit									15,300	15,800	12,500
Segment Assets	596,700				566,300				596,700	566,300	502,300
Capital Expenditures									64,900	62,300	61,100
Electricity
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									218,000	218,700	190,700
Interest Income									700	600	2,200
Interest Expense									8,800	9,100	9,500
Depreciation & Amortization Expense									24,000	22,300	20,300
Income Tax Expense (Benefit)									5,500	4,500	5,100
Segment Profit									8,700	6,800	7,600
Segment Assets	416,800				414,100				416,800	414,100	402,800
Capital Expenditures									29,900	24,800	23,600
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest Income									300	300	400
Interest Expense									1,700	1,600	1,500
Depreciation & Amortization Expense									900	1,000	1,000
Income Tax Expense (Benefit)									(1,100)	(1,900)	(700)
Segment Profit									1,000	1,200	300
Segment Assets	26,300				13,500				26,300	13,500	9,300
Capital Expenditures									9,000	5,200	4,800
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									6,200	5,700	5,800
Interest Income									100	100	100
Depreciation & Amortization Expense									100
Income Tax Expense (Benefit)									800	600	800
Segment Profit									1,300	900	1,200
Segment Assets	$ 6,600				$ 6,300				6,600	6,300	$ 6,200
Capital Expenditures									$ 100	$ 300

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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