Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) utility mi	Dec. 31, 2019 USD ($) mi	Dec. 31, 2018 USD ($)
Summary Of Accounting Policies [Line Items]
Number of Months to Establish ROU Asset and Liability	12 months
Operating Lease, Liability	$ 11,619
Number of Utilities that do not have cost recovery mechanism	1
Delay of Revenue Recognition Due To Implementation of New Standard | utility	2
Maturity Period To Be Considered Cash Equivalents	3 months
Contributions or Advances in Aid of Construction	$ 2,900	$ 2,100
Net property, plant and equipment	1,601,178	1,463,797
Accumulated depreciation	368,743	336,876
Depreciation and accretion reported in operations expenses	9,600	8,800	$ 8,500
Operating Lease, Right-of-Use Asset	11,194	$ 11,563
Natural Gas Operations
Summary Of Accounting Policies [Line Items]
Net property, plant and equipment	$ 6,700
Length of pipeline | mi	26	16
Minimum [Member]
Summary Of Accounting Policies [Line Items]
Operating Lease, Liability	$ 11,000
Maximum
Summary Of Accounting Policies [Line Items]
Operating Lease, Liability	$ 13,000