Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) mi	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	May 31, 2016 USD ($)
Summary Of Accounting Policies [Line Items]
Number of Months to Establish ROU Asset and Liability	12 months
Number of Utilities that do not have cost recovery mechanism	2
Delay of Revenue Recognition Due To Implementation of New Standard	$ (407)
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	$ (1,800)
Maturity Period To Be Considered Cash Equivalents	3
Contributions or Advances in Aid of Construction	$ 2,800	$ 2,100	$ 1,000
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	$ 720	652
Capital Leased Assets, Net	1,300	2,100
Capital Leased Assets, Noncurrent, Fair Value Disclosure	7,816	7,126
Ground Leases, Accumulated Amortization	6,506	5,056
Amortization of Leased Asset	1,451	1,401	1,353
Net property, plant and equipment	1,383,972	1,126,027
Accumulated depreciation	294,295	270,599
Depreciation and accretion reported in operations expenses	8,500	8,100	7,300
Gain from a settlement	(130)	(130)	$ (130)
Gain Contingency, Unrecorded Amount				$ 650
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	$ 1,363	$ 1,251
Minimum [Member]
Summary Of Accounting Policies [Line Items]
Operating Lease, Liability	11,000
Operating Lease, Right-of-Use Asset	11,000
Maximum
Summary Of Accounting Policies [Line Items]
Operating Lease, Liability	13,000
Operating Lease, Right-of-Use Asset	$ 13,000