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EMPLOYEE BENEFIT PLANS (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
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 3.25%   3.25% 3.35%  
Defined Benefit Plan Assumptions Used To Determine Benefit Obligations Rate of Compensation Increase, Maximum 3.40%   3.40% 3.40%  
Compensation increase rate - minimum 3.35% 3.35%   3.30% 3.35%
Compensation increase rate- maximum 3.40% 3.40%   3.50% 3.50%
Description of basis used to determine overall expected long-term rate of return on plan assets     We determine our overall expected long-term rate of return on plan assets, based on our review of historical returns and economic growth models. As of December 31, 2014, we updated our assumed mortality rates to incorporate the new set of mortality tables issued by the Society of Actuaries in October 2014. 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]          
Benefits paid $ (18,100)        
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Benefits paid (18,100)        
Noncurrent liabilities (303,507)   $ (303,507) $ (272,309)  
Benefits paid $ (18,100)        
Assets expected to be withdrawn or returned     0    
Amortization of unrecognized prior service cost     $ (3,316) (1,962) $ (1,211)
Amortization of loss     40,912 47,494 34,169
Total recognized in other comprehensive income (loss)     (314) $ 773 (1,781)
Amount recognized in other comprehensive income     $ 0    
Health care cost-trend rate assumed for next year     7.25%    
Rate to which the cost-trend rate is assumed to decline (the ultimate trend rate)     5.00%    
Year that the rate reaches the ultimate trend rate 2022   2022 2022  
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     37.40%    
Investment-grade bonds [Member]          
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Target asset allocation     30.00%    
Developed foreign large-cap equities [Member]          
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Target asset allocation     10.60%    
Alternative investments [Member]          
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Target asset allocation     7.70%    
Mid-cap equities [Member]          
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Target asset allocation     5.60%    
Emerging market equities [Member]          
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Target asset allocation     5.00%    
Small-cap equities [Member]          
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Target asset allocation     3.70%    
Equity Securities [Member]          
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]          
Defined Benefit Plan, Target Allocation Percentage     .7    
Fixed Income Funds [Member]          
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]          
Defined Benefit Plan, Target Allocation Percentage     .3    
Pension Plan [Member]          
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]          
Net periodic benefit cost     $ 31,965 $ 37,952 $ 27,071
Plan amendment     $ 0 $ 0  
Discount rate 4.30%   4.30% 4.75%  
Weighted average discount rate 4.75% 4.75%     5.25%
Expected long-term return on plan assets 7.75% 7.75%   7.75% 7.75%
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligation, beginning of period   $ 985,624 $ 985,624 $ 1,028,171  
Service cost     12,055 13,660 $ 11,620
Interest cost     45,550 43,542 43,791
Plan participants' contributions     0 0  
Actuarial loss (gain)     25,886 (47,607)  
Plan amendment     0 0  
Benefits paid     (71,066) (52,142)  
Settlements     (31,518) 0  
Benefit obligation, end of period $ 966,531   966,531 985,624 1,028,171
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets, beginning of period   785,161 785,161 845,396  
Actual return on plan assets     48,768 (9,026)  
Employer contributions     12,441 933  
Plan participants' contributions     0 0  
Benefits paid     (71,066) (52,142)  
Settlements     (35,718) 0  
Fair value of plan assets, end of period 739,586   739,586 785,161 845,396
Balance at December 31 (226,945)   (226,945) (200,463)  
Current liabilities (941)   (941) (912)  
Noncurrent liabilities (226,004)   (226,004) (199,551)  
Balance at December 31 (226,945)   (226,945) (200,463)  
Benefits paid     (71,066) (52,142)  
Accumulated benefit obligation 912,400   912,400 934,300  
Service cost     12,055 13,660 11,620
Interest cost     45,550 43,542 43,791
Expected return on assets     (61,183) (61,769) (59,862)
Amortization of unrecognized prior service cost     0 266 549
Amortization of loss     35,543 42,226 30,200
Settlements     0 27 773
Net periodic benefit cost     31,965 37,952 27,071
Net loss arising during the period     (1,262) 339 (3,543)
Amortization of loss     751 917 518
Deferred income taxes     197 (483) 1,244
Total recognized in other comprehensive income (loss)     314 (773) 1,781
Prior service credit (cost) 0   0 0  
Accumulated loss (414,757)   (414,757) (407,798)  
Accumulated other comprehensive loss before regulatory assets (414,757)   (414,757) (407,798)  
Regulatory asset for regulated entities 407,073   407,073 400,625  
Accumulated other comprehensive loss after regulatory assets (7,684)   (7,684) (7,173)  
Deferred income taxes 2,969   2,969 2,772  
Accumulated other comprehensive loss, net of tax (4,715)   (4,715) (4,401)  
Prior service credit (cost) expected to be recognized in following fiscal year 0   0    
Actuarial net loss, to be recognized in 2017 36,107   36,107    
Effect of a one percentage point change on total of service and interest cost - increase     233    
Effect of a one percentage point change on other postemployment benefit obligation - increase     3,937    
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]          
Level 3 fair value measurement, January 1   114,437 114,437 117,791  
Net realized and unrealized gains (losses)     3,898 2,246  
Settlements     (15,843) (5,600)  
Level 3 fair value measurement, December 31 102,492   102,492 114,437 117,791
Employer contributions     12,441 933  
Expected employer contributions in 2017     1,000    
2017 51,539   51,539    
2018 52,660   52,660    
2019 53,450   53,450    
2020 54,812   54,812    
2021 56,033   56,033    
2022 through 2027 294,519   294,519    
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   468,085 468,085    
Fair value of plan assets, end of period 430,642   430,642 468,085  
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   44,651 44,651    
Fair value of plan assets, end of period 47,445   47,445 44,651  
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   139,396 139,396    
Fair value of plan assets, end of period 129,036   129,036 139,396  
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   15,708 15,708    
Fair value of plan assets, end of period 29,900   29,900 15,708  
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   56,465 56,465    
Fair value of plan assets, end of period 45,140   45,140 56,465  
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]          
Level 3 fair value measurement, January 1   56,465 56,465 59,877  
Net realized and unrealized gains (losses)     4,518 2,188  
Settlements     (15,843) (5,600)  
Level 3 fair value measurement, December 31 45,140   45,140 56,465 59,877
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   60,856 60,856    
Fair value of plan assets, end of period 57,423   57,423 60,856  
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]          
Level 3 fair value measurement, January 1   57,972 57,972 57,914  
Net realized and unrealized gains (losses)     (620) 58  
Settlements     0 0  
Level 3 fair value measurement, December 31 57,352   57,352 57,972 57,914
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   414,248 414,248    
Fair value of plan assets, end of period 385,441   385,441 414,248  
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   405,935 405,935    
Fair value of plan assets, end of period 371,655   371,655 405,935  
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 0    
Fair value of plan assets, end of period 0   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 0    
Fair value of plan assets, end of period 0   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   5,429 5,429    
Fair value of plan assets, end of period 13,786   13,786 5,429  
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 0    
Fair value of plan assets, end of period 0   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   2,884 2,884    
Fair value of plan assets, end of period 0   0 2,884  
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   256,476 256,476    
Fair value of plan assets, end of period 251,653   251,653 256,476  
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   62,150 62,150    
Fair value of plan assets, end of period 58,987   58,987 62,150  
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   44,651 44,651    
Fair value of plan assets, end of period 47,445   47,445 44,651  
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   139,396 139,396    
Fair value of plan assets, end of period 129,036   129,036 139,396  
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   10,279 10,279    
Fair value of plan assets, end of period 16,114   16,114 10,279  
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 0    
Fair value of plan assets, end of period 0   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   0 0    
Fair value of plan assets, end of period 71   71 0  
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   114,437 114,437    
Fair value of plan assets, end of period 102,492   102,492 114,437  
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 0    
Fair value of plan assets, end of period 0   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 0    
Fair value of plan assets, end of period 0   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 0    
Fair value of plan assets, end of period 0   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 0    
Fair value of plan assets, end of period 0   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   56,465 56,465    
Fair value of plan assets, end of period 45,140   45,140 56,465  
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   $ 57,972 57,972    
Fair value of plan assets, end of period $ 57,352   57,352 57,972  
Other Postretirement Benefit Plan [Member]          
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]          
Net periodic benefit cost     $ 2,593 5,033 $ 5,889
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     5    
Plan amendment     $ 483 $ 11,901  
Discount rate 4.20%   4.20% 4.75%  
Weighted average discount rate 3.75% 4.75%     5.00%
Expected long-term return on plan assets 7.75% 8.00%   7.75% 7.75%
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligation, beginning of period   $ 228,253 $ 228,253 $ 257,688  
Service cost     2,675 3,257 $ 3,468
Interest cost     10,235 10,628 11,605
Plan participants' contributions     3,043 2,915  
Actuarial loss (gain)     14,309 (19,702)  
Plan amendment     483 11,901  
Benefits paid     (15,450) (14,632)  
Settlements     0 0  
Benefit obligation, end of period $ 243,548   243,548 228,253 257,688
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets, beginning of period   155,495 155,495 151,777  
Actual return on plan assets     9,733 1,335  
Employer contributions     13,225 14,100  
Plan participants' contributions     3,043 2,915  
Benefits paid     (15,450) (14,632)  
Settlements     0 0  
Fair value of plan assets, end of period 166,046   166,046 155,495 151,777
Balance at December 31 (77,502)   (77,502) (72,758)  
Current liabilities 0   0 0  
Noncurrent liabilities (77,502)   (77,502) (72,758)  
Balance at December 31 (77,502)   (77,502) (72,758)  
Benefits paid     (15,450) (14,632)  
Service cost     2,675 3,257 3,468
Interest cost     10,235 10,628 11,605
Expected return on assets     (12,370) (11,892) (11,393)
Amortization of unrecognized prior service cost     (3,316) (2,228) (1,760)
Amortization of loss     5,369 5,268 3,969
Net periodic benefit cost     2,593 5,033 5,889
Amortization of loss       0  
Prior service credit (cost) 10,211   10,211 14,010  
Accumulated loss (62,084)   (62,084) (50,447)  
Accumulated other comprehensive loss before regulatory assets (51,873)   (51,873) (36,437)  
Regulatory asset for regulated entities 51,873   51,873 36,437  
Accumulated other comprehensive loss after regulatory assets 0   0 0  
Deferred income taxes 0   0 0  
Accumulated other comprehensive loss, net of tax 0   0 0  
Prior service credit (cost) expected to be recognized in following fiscal year (4,597)   (4,597)    
Actuarial net loss, to be recognized in 2017 6,484   6,484    
Effect of a one percentage point change on total of service and interest cost - decrease     (232)    
Effect of a one percentage point change on other postemployment benefit obligation - decrease     (3,991)    
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]          
Employer contributions     13,225 14,100  
Expected employer contributions in 2017     3,100    
2017 16,165   16,165    
2018 16,815   16,815    
2019 17,073   17,073    
2020 17,379   17,379    
2021 17,401   17,401    
2022 through 2027 86,559   86,559    
Other Postretirement Benefit 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   62,058 62,058    
Fair value of plan assets, end of period 47,140   47,140 62,058  
Other Postretirement Benefit 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   64 64    
Fair value of plan assets, end of period 75   75 64  
Other Postretirement Benefit 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   200 200    
Fair value of plan assets, end of period 19,948   19,948 200  
Other Postretirement Benefit 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   13,555 13,555    
Fair value of plan assets, end of period 17,063   17,063 13,555  
Other Postretirement Benefit 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   79,531 79,531    
Fair value of plan assets, end of period 81,820   81,820 79,531  
Other Postretirement Benefit 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   87 87    
Fair value of plan assets, end of period       87  
Other Postretirement Benefit 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   54,797 54,797    
Fair value of plan assets, end of period 39,891   39,891 54,797  
Other Postretirement Benefit 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   54,560 54,560    
Fair value of plan assets, end of period 39,817   39,817 54,560  
Other Postretirement Benefit 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 0    
Fair value of plan assets, end of period 0   0 0  
Other Postretirement Benefit 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 0    
Fair value of plan assets, end of period 0   0 0  
Other Postretirement Benefit 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   233 233    
Fair value of plan assets, end of period 74   74 233  
Other Postretirement Benefit 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 0    
Fair value of plan assets, end of period 0   0 0  
Other Postretirement Benefit 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   4 4    
Fair value of plan assets, end of period       4  
Other Postretirement Benefit 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   100,615 100,615    
Fair value of plan assets, end of period 126,155   126,155 100,615  
Other Postretirement Benefit 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   7,498 7,498    
Fair value of plan assets, end of period 7,323   7,323 7,498  
Other Postretirement Benefit 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   64 64    
Fair value of plan assets, end of period 75   75 64  
Other Postretirement Benefit 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   200 200    
Fair value of plan assets, end of period 19,948   19,948 200  
Other Postretirement Benefit 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   13,322 13,322    
Fair value of plan assets, end of period 16,989   16,989 13,322  
Other Postretirement Benefit 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   79,531 79,531    
Fair value of plan assets, end of period 81,820   81,820 79,531  
Other Postretirement Benefit 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   0 0    
Fair value of plan assets, end of period       0  
Other Postretirement Benefit 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   83 83    
Fair value of plan assets, end of period 0   0 83  
Other Postretirement Benefit 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 0    
Fair value of plan assets, end of period 0   0 0  
Other Postretirement Benefit 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 0    
Fair value of plan assets, end of period 0   0 0  
Other Postretirement Benefit 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 0    
Fair value of plan assets, end of period 0   0 0  
Other Postretirement Benefit 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 0    
Fair value of plan assets, end of period 0   0 0  
Other Postretirement Benefit 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 0    
Fair value of plan assets, end of period $ 0   0 0  
Other Postretirement Benefit 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   $ 83 $ 83    
Fair value of plan assets, end of period       $ 83  
Maximum [Member]          
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Health care cost-trend rate assumed for next year       7.50%  
Rate to which the cost-trend rate is assumed to decline (the ultimate trend rate)       5.00%  
Maximum [Member] | Pension Plan [Member]          
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]          
Weighted average discount rate       4.75%  
Maximum [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]          
Weighted average discount rate       4.75%  
Minimum [Member]          
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Health care cost-trend rate assumed for next year       4.00%  
Rate to which the cost-trend rate is assumed to decline (the ultimate trend rate)       4.00%  
Minimum [Member] | Pension Plan [Member]          
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]          
Weighted average discount rate       4.25%  
Minimum [Member] | Other Postretirement Benefit Plan [Member]          
Defined Benefit Plans and Other Postemployment Benefit Plans Table Text Block [Line Items]          
Weighted average discount rate       4.25%  
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     $ 10,800 $ 10,200  
ONEOK 401(k) Plan [Member]          
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]          
Expense charged to us by ONEOK, Inc.         9,700
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     $ 6,000 $ 6,500  
ONEOK Profit-Sharing Plan [Member]          
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]          
Expense charged to us by ONEOK, Inc.         $ 4,000