Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) mi	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	May 01, 2013 USD ($)
Summary Of Accounting Policies [Line Items]
Maturity Period To Be Considered Cash Equivalents	3
Contributions or Advances in Aid of Construction	$ 1,000	$ 1,700
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	$ 580	507
Capital Leased Assets, Net	3,500	4,800
Capital Leased Assets, Noncurrent, Fair Value Disclosure				$ 7,100
Ground Leases, Accumulated Amortization	3,700	2,300
Amortization of Leased Asset	1,400	1,300
Net property, plant and equipment	986,664	854,950
Accumulated depreciation	245,207	215,313
Depreciation and accretion reported in operations expenses	$ 7,300	7,000	$ 6,600
Deferred revenue refund payment, period	1 year
(Gain from a settlement)/asset impairment charges	$ (130)	(1,500)	$ 6,881
Gain Contingency, Unrecorded Amount	650
Debt issuance costs	$ (291)	(333)
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	$ 15
Estimated decrease in annual pension and postretirement costs	(14)
Estimated change in annual post retirement and supplemental executive retirement plans	129
Natural Gas Operations
Summary Of Accounting Policies [Line Items]
Net property, plant and equipment	$ 6,670
Length of pipeline | mi	16
Ownership interest	45.00%
Accumulated depreciation	$ 1,029	$ 806
Regulated Energy
Summary Of Accounting Policies [Line Items]
(Gain from a settlement)/asset impairment charges	(6,400)
Chesapeake [Member]
Summary Of Accounting Policies [Line Items]
(Gain from a settlement)/asset impairment charges	$ 6,500