Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) utility mi	Dec. 31, 2018 USD ($) mi	Dec. 31, 2017 USD ($)
Summary Of Accounting Policies [Line Items]
Number of Months to Establish ROU Asset and Liability	12 months
Operating Lease, Liability	$ 11,601
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	$ 2,100	$ 2,800
Net property, plant and equipment	1,463,797	1,353,520
Accumulated depreciation	336,876	294,089
Depreciation and accretion reported in operations expenses	8,800	8,500	$ 8,100
Gain from a settlement	(130)	$ (130)	$ (130)
Operating Lease, Right-of-Use Asset	11,563
Natural Gas Operations
Summary Of Accounting Policies [Line Items]
Net property, plant and equipment	$ 6,700
Length of pipeline | mi	26	16
Ownership interest	45.00%
Accumulated depreciation	$ 1,500	$ 1,400
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