Background, Organization, and Summary of Significant Accounting Policies - Narratives (Details)	1 Months Ended			2 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2021 USD ($)	Mar. 31, 2021 USD ($)	Feb. 28, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2021 USD ($) state	May 31, 2021 USD ($)	Sep. 30, 2021 USD ($) state	Dec. 31, 2020 USD ($)	Nov. 30, 2018
Significant Accounting Policies [Line Items]
Number of states in which entity operates | state					3		3
Accounts receivable, extending period							1 month
Deferred purchased gas costs, recovered or refunded period							1 year
Deferred purchased gas costs, estimated incremental costs				$ 250,000,000
Deferred purchased gas costs, pipeline imbalance charges			$ 65,000,000			$ 55,000,000
Goodwill impairment							$ 0
Dividends declared but not yet paid					$ 35,900,000		35,900,000	$ 32,600,000
Self-insured retention amount					1,000,000		1,000,000
Threshold amount above self-insured retention					4,000,000		4,000,000
Loss contingency, maximum					5,000,000		5,000,000
Loss contingency accrual					5,000,000		5,000,000
Money Market Funds
Significant Accounting Policies [Line Items]
Money market fund investments					55,000,000		55,000,000	0
Subsequent Event | Questar Pipelines
Significant Accounting Policies [Line Items]
Payments to acquire business	$ 1,545,000,000
Assumption of long-term debt in business acquisition	$ 430,000,000
Linetec
Significant Accounting Policies [Line Items]
Ownership percentage by noncontrolling owners									20.00%
Gas Pipe Materials and Supplies | Prepaid and other current assets
Significant Accounting Policies [Line Items]
Gas pipe materials and operating supplies					55,000,000		$ 55,000,000	50,000,000
Southwest Gas Corporation
Significant Accounting Policies [Line Items]
Reclassification amount from Net utility plant to Prepaid and other current assets					$ 31,000,000
Southwest Gas Corporation | Money Market Funds
Significant Accounting Policies [Line Items]
Money market fund investments								$ 0
Southwest Gas Corporation | $250 Million Term Loan
Significant Accounting Policies [Line Items]
Debt instrument, term		364 days
Debt instrument, face amount		$ 250,000,000		$ 250,000,000