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EMPLOYEE BENEFIT PLANS (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]      
Regulatory Assets, Noncurrent $ 391,036,000 $ 437,479,000  
Defined Benefit Plan, Benefit Obligation, Payment for Settlement $ 47,200,000    
Defined Benefit Plan Assumptions Used To Determine Benefit Obligations Rate of Compensation Increase, Minimum 3.10% 3.20%  
Defined Benefit Plan Assumptions Used To Determine Benefit Obligations Rate of Compensation Increase, Maximum 4.00% 4.00%  
Compensation increase rate - minimum 3.20% 3.25% 3.25%
Compensation increase rate- maximum 4.00% 3.35% 3.40%
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.    
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Settlements $ 49,200,000    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Noncurrent liabilities (115,657,000) $ (178,720,000)  
Assets expected to be withdrawn or returned 0    
Amortization of unrecognized prior service cost (673,000) (4,567,000) $ (4,597,000)
Defined Benefit Plan, Amortization of Gain (Loss) 35,283,000 43,800,000 42,591,000
Total recognized in other comprehensive income (loss) (1,435,000) $ 1,407,000 (778,000)
Amount recognized in other comprehensive income $ 0    
Health care cost-trend rate assumed for next year 6.50% 7.00%  
Rate to which the cost-trend rate is assumed to decline (the ultimate trend rate) 5.00% 5.00%  
Year that the rate reaches the ultimate trend rate 2025 2024  
Target asset allocation 100.00%    
US Large-Cap Equity [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Target asset allocation 18.00%    
Investment-grade bonds [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Target asset allocation 40.00%    
Developed foreign large-cap equities [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Target asset allocation 10.00%    
Alternative investments [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Target asset allocation 14.00%    
Mid-cap equities [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Target asset allocation 7.00%    
Emerging market equities [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Target asset allocation 6.00%    
Small-cap equities [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Target asset allocation 5.00%    
Equity Securities [Member]      
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description 70 percent    
Fixed Income Funds [Member]      
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description 30 percent    
Pension Plan [Member]      
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant $ 0 $ 0  
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant 0 0  
Net periodic benefit cost $ 23,800,000 $ 29,054,000 $ 30,240,000
Discount rate 3.50% 4.40%  
Weighted average discount rate 4.40% 3.80% 4.30%
Expected long-term return on plan assets 7.20% 7.25% 7.75%
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation, beginning of period $ 950,510,000 $ 993,891,000  
Service cost 12,030,000 12,919,000 $ 12,176,000
Interest cost 40,670,000 36,801,000 40,453,000
Actuarial loss (gain) 98,231,000 (42,540,000)  
Benefits paid (50,915,000) (50,561,000)  
Settlements (49,158,000) 0  
Benefit obligation, end of period 1,001,368,000 950,510,000 993,891,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Actual return on plan assets 162,785,000 (62,752,000)  
Employer contributions 29,199,000 42,386,000  
Settlements 47,207,000 235,000  
Fair value of plan assets, end of period 907,974,000 814,112,000 884,804,000
Balance at December 31 (93,394,000) (136,398,000)  
Current liabilities (1,045,000) (962,000)  
Noncurrent liabilities (92,349,000) (135,436,000)  
Balance at December 31 (93,394,000) (136,398,000)  
Accumulated benefit obligation 937,800,000 890,400,000  
Service cost 12,030,000 12,919,000 12,176,000
Interest cost 40,670,000 36,801,000 40,453,000
Expected return on assets (61,939,000) (60,579,000) (58,496,000)
Amortization of unrecognized prior service cost 852,000 1,082,000 837,000
Defined Benefit Plan, Amortization of Gain (Loss) 33,039,000 39,913,000 36,107,000
Net loss arising during the period (2,766,000) 1,173,000 (2,101,000)
Deferred income taxes 479,000 (848,000) 486,000
Total recognized in other comprehensive income (loss) 1,435,000 (1,407,000) 778,000
Accumulated loss (381,633,000) (419,238,000)  
Accumulated other comprehensive loss before regulatory assets (381,633,000) (419,238,000)  
Regulatory asset for regulated entities 373,025,000 412,545,000  
Accumulated other comprehensive loss after regulatory assets (8,608,000) (6,693,000)  
Deferred income taxes 1,869,000 2,607,000  
Accumulated other comprehensive loss, net of tax (6,739,000) (4,086,000)  
Prior service credit (cost) expected to be recognized in following fiscal year 0    
Actuarial net loss, to be recognized in 2017 42,319,000    
Effect of a one percentage point change on total of service and interest cost - increase 100,000    
Effect of a one percentage point change on other postemployment benefit obligation - increase 2,300,000    
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]      
Level 3 fair value measurement, January 1 109,650,000 113,865,000  
Net realized and unrealized gains (losses) 1,728,000 (115,000)  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 0 0  
Settlements (3,597,000) (4,100,000)  
Level 3 fair value measurement, December 31 107,781,000 109,650,000 113,865,000
Employer contributions 29,199,000 42,386,000  
Expected employer contributions in 2017 1,100,000    
2017 49,631,000    
2018 50,378,000    
2019 51,547,000    
2020 52,628,000    
2021 53,562,000    
2022 through 2027 280,547,000    
Defined Benefit Plan, Benefit Obligation, Benefits Paid (50,915,000) (50,561,000)  
Pension Plan [Member] | Equity Securities [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets, end of period 351,004,000 318,538,000  
Pension Plan [Member] | Government Obligations [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets, end of period 54,726,000 69,475,000  
Pension Plan [Member] | Corporate Obligations [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets, end of period 304,457,000 240,900,000  
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, end of period 89,109,000 74,410,000  
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, end of period 25,988,000 30,445,000  
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]      
Level 3 fair value measurement, January 1 30,445,000 35,158,000  
Net realized and unrealized gains (losses) (860,000) (611,000)  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 0 0  
Settlements (3,597,000) (4,100,000)  
Level 3 fair value measurement, December 31 25,988,000 30,445,000 35,158,000
Pension Plan [Member] | Other Investments [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets, end of period 82,690,000 80,344,000  
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]      
Level 3 fair value measurement, January 1 79,205,000 78,707,000  
Net realized and unrealized gains (losses) 2,588,000 496,000  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 0 0  
Settlements 0 0  
Level 3 fair value measurement, December 31 81,793,000 79,205,000 78,707,000
Pension Plan [Member] | Level 1 [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets, end of period 325,424,000 285,087,000  
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, end of period 323,737,000 282,668,000  
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, 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, 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, end of period 1,687,000 2,419,000  
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, 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, 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, end of period 474,769,000 419,375,000  
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, end of period 27,267,000 35,870,000  
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, end of period 54,726,000 69,475,000  
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, end of period 304,457,000 240,900,000  
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, end of period 87,422,000 71,991,000  
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, 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, end of period 897,000 1,139,000  
Pension Plan [Member] | Level 3 [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets, end of period 107,781,000 109,650,000  
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, 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, 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, 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, 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, end of period   30,445,000  
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, end of period   79,205,000  
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant 3,697,000 3,563,000  
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant 3,697,000 3,563,000  
Net periodic benefit cost $ 37,000 $ (3,493,000) $ 1,696,000
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    
Discount rate 3.40% 4.40%  
Weighted average discount rate 4.40% 3.70% 4.20%
Expected long-term return on plan assets 7.35% 7.60% 7.60%
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation, beginning of period $ 220,144,000 $ 255,040,000  
Service cost 1,734,000 2,354,000 $ 2,509,000
Interest cost 9,318,000 9,117,000 9,890,000
Actuarial loss (gain) 13,945,000 (31,607,000)  
Benefits paid (18,348,000) (18,323,000)  
Settlements 0 0  
Benefit obligation, end of period 230,490,000 220,144,000 255,040,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Actual return on plan assets 38,772,000 (6,325,000)  
Employer contributions 6,202,000 7,718,000  
Settlements 0 0  
Fair value of plan assets, end of period 207,182,000 176,859,000 190,226,000
Balance at December 31 (23,308,000) (43,285,000)  
Current liabilities 0 0  
Noncurrent liabilities (23,308,000) (43,285,000)  
Balance at December 31 (23,308,000) (43,285,000)  
Service cost 1,734,000 2,354,000 2,509,000
Interest cost 9,318,000 9,117,000 9,890,000
Expected return on assets (12,586,000) (14,284,000) (12,590,000)
Amortization of unrecognized prior service cost (673,000) (4,567,000) (4,597,000)
Defined Benefit Plan, Amortization of Gain (Loss) 2,244,000 3,887,000 6,484,000
Prior service credit (cost) 202,000 875,000  
Accumulated loss (19,660,000) (34,144,000)  
Accumulated other comprehensive loss before regulatory assets (19,458,000) (33,269,000)  
Regulatory asset for regulated entities 19,458,000 33,269,000  
Accumulated other comprehensive loss after regulatory assets 0 0  
Prior service credit (cost) expected to be recognized in following fiscal year (117,000)    
Actuarial net loss, to be recognized in 2017 173,000    
Effect of a one percentage point change on total of service and interest cost - decrease (100,000)    
Effect of a one percentage point change on other postemployment benefit obligation - decrease (2,400,000)    
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]      
Employer contributions 6,202,000 7,718,000  
Expected employer contributions in 2017 4,000,000.0    
2017 16,464,000    
2018 16,308,000    
2019 16,269,000    
2020 16,090,000    
2021 15,757,000    
2022 through 2027 73,787,000    
Defined Benefit Plan, Benefit Obligation, Benefits Paid (18,348,000) (18,323,000)  
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, end of period 61,688,000 60,469,000  
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, end of period 0 74,000  
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, end of period 26,852,000 25,857,000  
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, end of period 19,032,000 1,549,000  
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, end of period 99,610,000 88,910,000  
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, end of period 80,038,000 59,336,000  
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, end of period 61,688,000 58,087,000  
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, 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, 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, end of period 18,350,000 1,249,000  
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, 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, end of period 127,144,000 117,523,000  
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, end of period 0 2,382,000  
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, end of period 0 74,000  
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, end of period 26,852,000 25,857,000  
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, end of period 682,000 300,000  
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, end of period 99,610,000 88,910,000  
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, 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, 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, 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, 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, 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, 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 $ 12,800,000 12,100,000 11,700,000
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 $ 8,500,000 7,400,000 $ 8,100,000
Non-service Costs [Member]      
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]      
Regulatory Assets, Noncurrent $ 4,700,000 $ 1,500,000