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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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.

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The total amount of operating revenues recognized during the period.

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Amount of operating expense for depreciation and amortization of regulated operation.

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Significant Segment Financial Data (Detail) (USD $)	3 Months Ended								12 Months Ended
Significant Segment Financial Data (Detail) (USD $)	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	$ 119,800,000	$ 76,600,000	$ 73,300,000	$ 156,100,000	$ 109,800,000	$ 72,500,000	$ 66,400,000	$ 118,200,000	$ 425,800,000	$ 366,900,000	$ 353,100,000
Interest Income									1,300,000us-gaap_InvestmentIncomeInterest	3,200,000us-gaap_InvestmentIncomeInterest	4,200,000us-gaap_InvestmentIncomeInterest
Interest Expense									22,200,000us-gaap_InterestExpenseDebt	22,000,000us-gaap_InterestExpenseDebt	22,300,000us-gaap_InterestExpenseDebt
Depreciation & Amortization Expense									42,100,000	38,500,000	36,000,000
Income Tax Expense (Benefit)									13,995,000us-gaap_IncomeTaxExpenseBenefit	12,652,000us-gaap_IncomeTaxExpenseBenefit	10,976,000us-gaap_IncomeTaxExpenseBenefit
Segment Profit	9,400,000	1,600,000	1,100,000	12,600,000	10,300,000	600,000	(100,000)	10,800,000	24,700,000	21,600,000	18,100,000
Identifiable Segment Assets	1,000,200,000us-gaap_Assets				920,600,000us-gaap_Assets				1,000,200,000us-gaap_Assets	920,600,000us-gaap_Assets	892,300,000us-gaap_Assets
Capital Expenditures									92,600,000	89,500,000	68,500,000
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									201,400,000	170,400,000	160,600,000
Interest Income									300,000	500,000	800,000
Interest Expense									11,500,000	11,000,000	11,100,000
Depreciation & Amortization Expense									18,800,000	17,200,000	15,700,000
Income Tax Expense (Benefit)									10,800,000	7,500,000	5,800,000
Segment Profit									15,800,000	12,500,000	8,900,000
Identifiable Segment Assets	566,300,000				502,300,000				566,300,000	502,300,000	471,700,000
Capital Expenditures									62,300,000	61,100,000	43,900,000
Electricity
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									218,700,000	190,700,000	187,000,000
Interest Income									600,000	2,200,000	2,900,000
Interest Expense									9,100,000	9,500,000	9,500,000
Depreciation & Amortization Expense									22,300,000	20,300,000	18,900,000
Income Tax Expense (Benefit)									4,500,000	5,100,000	4,800,000
Segment Profit									6,800,000	7,600,000	7,600,000
Identifiable Segment Assets	414,100,000				402,800,000				414,100,000	402,800,000	409,300,000
Capital Expenditures									24,800,000	23,600,000	21,200,000
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest Income									300,000	400,000	400,000
Interest Expense									1,600,000	1,500,000	1,700,000
Depreciation & Amortization Expense									1,000,000	1,000,000	1,400,000
Income Tax Expense (Benefit)									(1,900,000)	(700,000)	(500,000)
Segment Profit									1,200,000	300,000	300,000
Identifiable Segment Assets	13,500,000				9,300,000				13,500,000	9,300,000	5,600,000
Capital Expenditures									5,200,000	4,800,000	3,400,000
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									5,700,000	5,800,000	5,500,000
Interest Income									100,000	100,000	100,000
Income Tax Expense (Benefit)									600,000	800,000	900,000
Segment Profit									900,000	1,200,000	1,300,000
Identifiable Segment Assets	6,300,000				6,200,000				6,300,000	6,200,000	5,700,000
Capital Expenditures									$ 300,000