Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) mi	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2009 USD ($)
Summary Of Accounting Policies [Line Items]
Unamortized Debt Issuance Expense	$ 358
Maturity Period To Be Considered Cash Equivalents	3
Inventory Write-down	$ 681
Non-refundable contributions for construction received	1,700	$ 813
Allowed funds used during construction	38	58	$ 131
Property, plant and equipment in service assets under operating lease contract	1,400
Annual revenue from operating lease	$ 264
Operating lease term	20 years
Accumulated depreciation, plant in service assets under operating lease	$ 507	435
Capital Leased Assets, Gross	4,800	6,100
Ground Leases, Accumulated Amortization	2,300	996
Amortization of Leased Asset	1,300	848
Net property, plant and equipment	854,950	689,762
Accumulated depreciation	215,313	193,369
Gain From A Settlement	1,500
Depreciation and accretion reported in operations expenses	$ 7,000	6,600	6,100
Deferred revenue refund payment, period	1 year
Asset Impairment Charges	$ (1,500)	6,881	$ 0
Gain Contingency, Unrecorded Amount	$ 750
Period Additional Cash And Discounts On Future Services Is Based On	5 years
Pensions and other employee benefits
Summary Of Accounting Policies [Line Items]
Percentage of change in discount rate	0.25%
Percentage of change in rate of return	0.25%
Estimated increase in annual pension and postretirement costs	$ 19
Estimated decrease in annual pension and postretirement costs	(19)
Estimated change in annual post retirement and supplemental executive retirement plans	126
Natural Gas Operations
Summary Of Accounting Policies [Line Items]
Net property, plant and equipment	$ 6,700
Length of pipeline | mi	16
Ownership interest	4500.00%
Accumulated depreciation	$ 806	$ 584
Parent
Summary Of Accounting Policies [Line Items]
Premium paid for acquisition				$ 746	$ 34,200
Regulated Energy
Summary Of Accounting Policies [Line Items]
Asset Impairment Charges	6,400
Unregulated Energy
Summary Of Accounting Policies [Line Items]
Asset Impairment Charges	19
Chesapeake [Member]
Summary Of Accounting Policies [Line Items]
Asset Impairment Charges	6,500
Noncompete Agreements [Member]
Summary Of Accounting Policies [Line Items]
Impairment of Intangible Assets (Excluding Goodwill)	$ 175