EMPLOYEE BENEFIT PLANS (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan Assumptions Used To Determine Benefit Obligations Rate of Compensation Increase, Minimum	3.10%	3.10%
Defined Benefit Plan Assumptions Used To Determine Benefit Obligations Rate of Compensation Increase, Maximum	5.00%	3.90%
Compensation increase rate - minimum	3.10%	3.10%	3.20%
Compensation increase rate- maximum	3.90%	4.00%	4.00%
Description of basis used to determine overall expected long-term rate of return on plan assets	We determine our discount rates annually. We estimate our discount rate based upon a comparison of the expected cash flows associated with our future payments under our defined benefit pension and other postemployment obligations to a hypothetical bond portfolio created using high-quality bonds that closely match expected cash flows. Bond portfolios are developed by selecting a bond for each of the next 60 years based on the maturity dates of the bonds. Bonds selected to be included in the portfolios are only those rated by Moody’s as AA- or better and exclude callable bonds, bonds with less than a minimum issue size, yield outliers and other filtering criteria to remove unsuitable bonds.
Regulatory Assets, Noncurrent	$ 724,862,000	$ 366,956,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Liability, Defined Benefit Plan, Noncurrent	(35,226,000)	(97,637,000)
Lump Sum benefit payments paid		12,500,000
Amortization of unrecognized prior service cost	(279,000)	(117,000)	$ (673,000)
Defined Benefit Plan, Amortization of Gain (Loss)	45,896,000	42,492,000	35,283,000
Total recognized in other comprehensive income (loss)	1,250,000	$ (1,038,000)	$ (1,435,000)
Amount recognized in other comprehensive income	$ 0
Health care cost-trend rate assumed for next year	6.00%	6.25%
Rate to which the cost-trend rate is assumed to decline (the ultimate trend rate)	4.50%	4.50%
Year that the rate reaches the ultimate trend rate	2028	2026
Target asset allocation	100.00%
Grosvenor Registered Multi Limited Partnership
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	$ 42,632,000
Fair value of plan assets, end of period	44,818,000	$ 42,632,000
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 0	$ 0
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Frequency	quarterly	quarterly
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Description	65	65
K2 Institutional Investors II Limited Partnership
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	$ 45,002,000
Fair value of plan assets, end of period	46,638,000	$ 45,002,000
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 0	$ 0
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Frequency	quarterly	quarterly
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Description	91	91
Investment-grade bonds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Target asset allocation	60.00%
US Large-Cap Equity [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Target asset allocation	14.00%
Alternative investments [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Target asset allocation	10.00%
Developed foreign large-cap equities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Target asset allocation	7.00%
Mid-cap equities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Target asset allocation	5.00%
Emerging market equities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Target asset allocation	1.00%
Small-cap equities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Target asset allocation	3.00%
Fixed Income Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description	65 percent
Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description	35 percent
Pension Plan [Member]
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]
Discount rate	3.05%	2.80%
Weighted average discount rate	2.80%	3.50%	4.40%
Expected long-term return on plan assets	7.15%	7.20%	7.20%
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	$ 0	$ 0
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of period	1,077,641,000	1,001,368,000
Service cost	13,811,000	12,869,000	$ 12,030,000
Interest cost	29,458,000	34,179,000	40,670,000
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	0	0
Actuarial loss (gain)	(19,587,000)	91,566,000
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(51,333,000)	(62,341,000)
Benefit obligation, end of period	1,049,990,000	1,077,641,000	1,001,368,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	987,583,000	907,974,000
Actual return on plan assets	75,999,000	140,939,000
Employer contributions	995,000	1,011,000
Benefits paid	(51,333,000)	(62,341,000)
Settlements	0	0
Fair value of plan assets, end of period	1,013,244,000	987,583,000	907,974,000
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(36,746,000)	(90,058,000)
Assets for Plan Benefits, Defined Benefit Plan	0	0
Liability, Defined Benefit Plan, Current	(1,521,000)	(1,056,000)
Liability, Defined Benefit Plan, Noncurrent	(35,225,000)	(89,002,000)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position	(36,746,000)	(90,058,000)
Accumulated benefit obligation	1,000,000,000	1,000,000,000
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	1,500,000
Service cost	13,811,000	12,869,000	12,030,000
Interest cost	29,458,000	34,179,000	40,670,000
Expected return on assets	(62,382,000)	(61,119,000)	(61,939,000)
Amortization of unrecognized prior service cost	1,562,000	1,192,000	852,000
Defined Benefit Plan, Amortization of Gain (Loss)	45,523,000	42,319,000	33,039,000
Net periodic benefit cost	26,410,000	28,248,000	23,800,000
Net loss arising during the period	67,000	(2,519,000)	(2,766,000)
Deferred income taxes	(379,000)	289,000	479,000
Total recognized in other comprehensive income (loss)	1,250,000	(1,038,000)	(1,435,000)
Accumulated loss	(272,332,000)	(351,059,000)
Accumulated other comprehensive loss before regulatory assets	(272,332,000)	(351,059,000)
Regulatory asset for regulated entities	264,027,000	341,125,000
Accumulated other comprehensive loss after regulatory assets	(8,305,000)	(9,934,000)
Deferred income taxes	1,778,000	2,157,000
Accumulated other comprehensive loss, net of tax	(6,527,000)	(7,777,000)
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Level 3 fair value measurement, January 1	112,237,000	107,781,000
Net realized and unrealized gains (losses)		6,613,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Period Increase	1,625,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Period Decrease	(3,368,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	2,197,000	992,000
Settlements	(3,934,000)	(3,149,000)
Level 3 fair value measurement, December 31	108,757,000	112,237,000	107,781,000
2021	52,936,000
2022	53,745,000
2023	54,440,000
2024	55,075,000
2025	55,852,000
2026 through 2030	283,344,000
Pension Plan [Member] | Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	428,093,000
Fair value of plan assets, end of period	223,871,000	428,093,000
Pension Plan [Member] | Government Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	78,080,000
Fair value of plan assets, end of period	205,741,000	78,080,000
Pension Plan [Member] | Corporate Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	343,118,000
Fair value of plan assets, end of period	440,445,000	343,118,000
Pension Plan [Member] | Cash and Money Market Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	24,900,000
Fair value of plan assets, end of period	34,410,000	24,900,000
Pension Plan [Member] | Insurance contracts and group annuity contracts [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	24,603,000
Fair value of plan assets, end of period	17,301,000	24,603,000
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Level 3 fair value measurement, January 1	24,603,000	25,988,000
Net realized and unrealized gains (losses)		1,764,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Period Increase	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Period Decrease	(3,368,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0
Settlements	(3,934,000)	(3,149,000)
Level 3 fair value measurement, December 31	17,301,000	24,603,000	25,988,000
Pension Plan [Member] | Other Investments [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	88,789,000
Fair value of plan assets, end of period	91,476,000	88,789,000
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Level 3 fair value measurement, January 1	87,634,000	81,793,000
Net realized and unrealized gains (losses)		4,849,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Period Increase	1,625,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Period Decrease	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	2,197,000	992,000
Settlements	0	0
Level 3 fair value measurement, December 31	91,456,000	87,634,000	$ 81,793,000
Pension Plan [Member] | Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	394,228,000
Fair value of plan assets, end of period	227,735,000	394,228,000
Pension Plan [Member] | Level 1 [Member] | Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	392,639,000
Fair value of plan assets, end of period	223,871,000	392,639,000
Pension Plan [Member] | Level 1 [Member] | Government Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Pension Plan [Member] | Level 1 [Member] | Corporate Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Pension Plan [Member] | Level 1 [Member] | Cash and Money Market Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	1,589,000
Fair value of plan assets, end of period	3,864,000	1,589,000
Pension Plan [Member] | Level 1 [Member] | Insurance contracts and group annuity contracts [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Pension Plan [Member] | Level 1 [Member] | Other Investments [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Pension Plan [Member] | Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	481,118,000
Fair value of plan assets, end of period	676,752,000	481,118,000
Pension Plan [Member] | Level 2 [Member] | Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	35,454,000
Fair value of plan assets, end of period	0	35,454,000
Pension Plan [Member] | Level 2 [Member] | Government Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	78,080,000
Fair value of plan assets, end of period	205,741,000	78,080,000
Pension Plan [Member] | Level 2 [Member] | Corporate Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	343,118,000
Fair value of plan assets, end of period	440,445,000	343,118,000
Pension Plan [Member] | Level 2 [Member] | Cash and Money Market Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	23,311,000
Fair value of plan assets, end of period	30,546,000	23,311,000
Pension Plan [Member] | Level 2 [Member] | Insurance contracts and group annuity contracts [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Pension Plan [Member] | Level 2 [Member] | Other Investments [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	1,155,000
Fair value of plan assets, end of period	20,000	1,155,000
Pension Plan [Member] | Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	112,237,000
Fair value of plan assets, end of period	108,757,000	112,237,000
Pension Plan [Member] | Level 3 [Member] | Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Pension Plan [Member] | Level 3 [Member] | Government Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Pension Plan [Member] | Level 3 [Member] | Corporate Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Pension Plan [Member] | Level 3 [Member] | Cash and Money Market Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Pension Plan [Member] | Level 3 [Member] | Insurance contracts and group annuity contracts [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	24,603,000
Fair value of plan assets, end of period		24,603,000
Pension Plan [Member] | Level 3 [Member] | Other Investments [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	$ 87,634,000
Fair value of plan assets, end of period		$ 87,634,000
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]
Minimum number of years of service for certain employees to be eligible to participate in shared welfare plans that provide postemployment medical and life insurance benefits	five years
Discount rate	3.00%	2.70%
Weighted average discount rate	2.70%	3.40%	4.40%
Expected long-term return on plan assets	7.50%	7.65%	7.35%
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	$ 3,226,000	$ 3,500,000
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of period	239,530,000	230,490,000
Service cost	1,587,000	1,692,000	$ 1,734,000
Interest cost	6,251,000	7,557,000	9,318,000
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	3,226,000	3,500,000
Actuarial loss (gain)	(8,894,000)	14,013,000
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(18,894,000)	(17,722,000)
Benefit obligation, end of period	222,806,000	239,530,000	230,490,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	230,895,000	207,182,000
Actual return on plan assets	14,786,000	35,837,000
Employer contributions	1,981,000	2,098,000
Benefits paid	(18,894,000)	(17,722,000)
Settlements	0	0
Fair value of plan assets, end of period	231,994,000	230,895,000	207,182,000
Defined Benefit Plan, Funded (Unfunded) Status of Plan	9,188,000	(8,635,000)
Assets for Plan Benefits, Defined Benefit Plan	9,188,000	0
Liability, Defined Benefit Plan, Current	0	0
Liability, Defined Benefit Plan, Noncurrent	0	(8,635,000)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position	9,188,000	(8,635,000)
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	0
Service cost	1,587,000	1,692,000	1,734,000
Interest cost	6,251,000	7,557,000	9,318,000
Expected return on assets	(16,807,000)	(15,469,000)	(12,586,000)
Amortization of unrecognized prior service cost	(279,000)	(117,000)	(673,000)
Defined Benefit Plan, Amortization of Gain (Loss)	373,000	173,000	2,244,000
Net periodic benefit cost	(8,875,000)	(6,164,000)	37,000
Prior service credit (cost)	(194,000)	85,000
Accumulated loss	(5,887,000)	(13,134,000)
Accumulated other comprehensive loss before regulatory assets	(6,081,000)	(13,049,000)
Regulatory asset for regulated entities	6,081,000	13,049,000
Accumulated other comprehensive loss after regulatory assets	0	0
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
2021	15,744,000
2022	15,571,000
2023	15,184,000
2024	14,940,000
2025	14,580,000
2026 through 2030	67,566,000
Other Postretirement Benefits Plan [Member] | Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	73,578,000
Fair value of plan assets, end of period	25,577,000	73,578,000
Other Postretirement Benefits Plan [Member] | Government Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	41,366,000	0
Other Postretirement Benefits Plan [Member] | Corporate Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	39,115,000
Fair value of plan assets, end of period	41,601,000	39,115,000
Other Postretirement Benefits Plan [Member] | Cash and Money Market Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	8,123,000
Fair value of plan assets, end of period	13,532,000	8,123,000
Other Postretirement Benefits Plan [Member] | Insurance contracts and group annuity contracts [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	110,079,000
Fair value of plan assets, end of period	109,918,000	110,079,000
Other Postretirement Benefits Plan [Member] | Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	73,630,000
Fair value of plan assets, end of period	26,119,000	73,630,000
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	73,578,000
Fair value of plan assets, end of period	25,577,000	73,578,000
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | Government Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | Corporate Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | Cash and Money Market Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	52,000
Fair value of plan assets, end of period	542,000	52,000
Other Postretirement Benefits Plan [Member] | Level 1 [Member] | Insurance contracts and group annuity contracts [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Other Postretirement Benefits Plan [Member] | Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	157,265,000
Fair value of plan assets, end of period	205,875,000	157,265,000
Other Postretirement Benefits Plan [Member] | Level 2 [Member] | Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Other Postretirement Benefits Plan [Member] | Level 2 [Member] | Government Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	41,366,000	0
Other Postretirement Benefits Plan [Member] | Level 2 [Member] | Corporate Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	39,115,000
Fair value of plan assets, end of period	41,601,000	39,115,000
Other Postretirement Benefits Plan [Member] | Level 2 [Member] | Cash and Money Market Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	8,071,000
Fair value of plan assets, end of period	12,990,000	8,071,000
Other Postretirement Benefits Plan [Member] | Level 2 [Member] | Insurance contracts and group annuity contracts [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	110,079,000
Fair value of plan assets, end of period	109,918,000	110,079,000
Other Postretirement Benefits Plan [Member] | Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Other Postretirement Benefits Plan [Member] | Level 3 [Member] | Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Other Postretirement Benefits Plan [Member] | Level 3 [Member] | Government Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Other Postretirement Benefits Plan [Member] | Level 3 [Member] | Corporate Obligations [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Other Postretirement Benefits Plan [Member] | Level 3 [Member] | Cash and Money Market Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	0	0
Other Postretirement Benefits Plan [Member] | Level 3 [Member] | Insurance contracts and group annuity contracts [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of period	0
Fair value of plan assets, end of period	$ 0	0
ONE Gas 401(k) Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Percent of employee contributions matched of eligible compensation (in hundredths)	100.00%
Maximum percentage of each participants eligible compensation subject to certain limits matching (in hundredths)	6.00%
Contributions made to 401(k) plan	$ 14,300,000	13,800,000	12,800,000
ONE Gas Profit-Sharing Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Profit sharing contribution percentage	1.00%
Contributions made to profit-sharing plan	$ 9,900,000	9,400,000	$ 8,500,000
Non-service Costs [Member]
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]
Regulatory Assets, Noncurrent	$ 6,100,000	$ 6,000,000