Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) mi	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	May 31, 2016 USD ($)	May 01, 2013 USD ($)
Summary Of Accounting Policies [Line Items]
Number of Utilities that do not have cost recovery mechanism	2
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 1,500,000
Number of contracts affecting new revenue recognition principle adoption	1
Delay of Revenue Recognition Due To Implementation of New Standard	$ 407,000
Maturity Period To Be Considered Cash Equivalents	3
Contributions or Advances in Aid of Construction	$ 2,100,000	$ 1,000,000	$ 1,700,000
Property, plant and equipment in service assets under operating lease contract	1,400,000
Annual revenue from operating lease	$ 264,000
Operating lease term	20 years
Accumulated depreciation, plant in service assets under operating lease	$ 652,000	580,000
Capital Leased Assets, Net	2,000,000	3,400,000
Capital Leased Assets, Noncurrent, Fair Value Disclosure					$ 7,100,000
Ground Leases, Accumulated Amortization	5,100,000	3,700,000
Amortization of Leased Asset	1,400,000	1,400,000	1,300,000
Net property, plant and equipment	1,126,027,000	986,664,000
Accumulated depreciation	270,599,000	245,207,000
Depreciation and accretion reported in operations expenses	$ 8,100,000	7,300,000	7,000,000
Deferred revenue refund payment, period	1 year
Gain from a settlement	$ (130,000)	(130,000)	$ (1,500,000)
Gain Contingency, Unrecorded Amount				$ 650,000
Natural Gas Operations
Summary Of Accounting Policies [Line Items]
Net property, plant and equipment	$ 6,670,000
Length of pipeline | mi	16
Ownership interest	45.00%
Accumulated depreciation	$ 1,251,000	$ 1,029,000
Pension And Other Postretirement Benefits [Member]
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	$ 7,000
Estimated decrease in annual pension and postretirement costs	(9,000)
Estimated change in annual post retirement and supplemental executive retirement plans	$ 143,000