XML 106 R82.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Retirement Plans and Other Benefits - Weighted-Average Assumptions for Pensions and Other Benefits (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Weighted-average assumptions used to determine benefit obligations |  |  |
Rate of compensation increase (as a percent) | 4.00% | 4.00% |
Initial pre-65 healthcare cost trend rate (as a percent) | 7.00% | 7.00% |
Initial post-65 healthcare cost trend rate (as a percent) | 4.75% | 4.75% |
Ultimate health care cost trend rate (as a percent) | 4.75% | 4.75% |
Number of years to ultimate trend rate (pre-65 participants) | 7 years | 8 years |
Weighted-average assumptions used to determine net periodic benefit costs |  |  |
Initial pre-65 health care cost trend rate (as a percent) | 7.00% | 7.00% | 7.00%
Initial post-65 health care cost trend rate (as a percent) | 4.75% | 5.00% | 5.00%
Ultimate healthcare cost trend rate (as a percent) | 4.75% | 5.00% | 5.00%
Number of years to ultimate trend rate (pre-65 participants) | 8 years | 4 years | 4 years
Pension Benefits |  |  |
Weighted-average assumptions used to determine benefit obligations |  |  |
Discount rate (as a percent) | 4.34% | 3.65% |
Weighted-average assumptions used to determine net periodic benefit costs |  |  |
Discount rate (as a percent) | 3.65% | 4.08% | 4.37%
Rate of compensation increase (as a percent) | 4.00% | 4.00% | 4.00%
Expected long-term return on plan assets (as a percent) | 6.05% | 6.55% | 6.90%
Other Benefits |  |  |
Weighted-average assumptions used to determine benefit obligations |  |  |
Discount rate (as a percent) | 4.39% | 3.71% |
Weighted-average assumptions used to determine net periodic benefit costs |  |  |
Discount rate (as a percent) | 3.71% | 4.17% | 4.52%
Expected long-term return on plan assets (as a percent) | 5.40% | 6.05% | 4.45%
Effects of one percentage point change in the assumed initial and ultimate health care cost trend rates |  |  |
Effect of 1% increase on other postretirement benefits expense, after consideration of amounts capitalized or billed to electric plant participants | $ 10,235 |  |
Effect of 1% decrease on other postretirement benefits expense, after consideration of amounts capitalized or billed to electric plant participants | (4,322) |  |
Effect of 1% increase on service and interest cost components of net periodic other postretirement benefit costs | 11,223 |  |
Effect of 1% decrease on service and interest cost components of net periodic other postretirement benefit costs | (8,479) |  |
Effect of 1% increase on the accumulated other postretirement benefit obligation | 101,224 |  |
Effect of 1% decrease on the accumulated other postretirement benefit obligation | $ (81,144) |  |

X
- Definition The effect of a one-percentage-point decrease in the assumed health care cost trend rates on the other postretirement benefits expense, after consideration of amount capitalized or billed to electric plant participants. + References No definition available. + Details
Name: | pnw_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnOtherPostretirementBenefitsExpense
Namespace Prefix: | pnw_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The effect of a one-percentage-point increase in the assumed health care cost trend rates on the other postretirement benefits expense, after consideration of amount capitalized or billed to electric plant participants. + References No definition available. + Details
Name: | pnw_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnOtherPostretirementBenefitsExpense
Namespace Prefix: | pnw_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Defined Benefit Plan, Post-65 Health Care Cost Trend Rate Assumed for Next Fiscal Year, Used in Calculating Benefit Costs + References No definition available. + Details
Name: | pnw_DefinedBenefitPlanPost65HealthCareCostTrendRateAssumedforNextFiscalYearUsedinCalculatingBenefitCosts
Namespace Prefix: | pnw_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Defined Benefit Plan, Post-65 Health Care Cost Trend Rate Assumed for Next Fiscal Year, Used in Calculating Benefit Obligation + References No definition available. + Details
Name: | pnw_DefinedBenefitPlanPost65HealthCareCostTrendRateAssumedforNextFiscalYearUsedinCalculatingBenefitObligation
Namespace Prefix: | pnw_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Defined Benefit Plan, Pre-65 Health Care Cost Trend Rate Assumed for Next Fiscal Year + References No definition available. + Details
Name: | pnw_DefinedBenefitPlanPre65HealthCareCostTrendRateAssumedforNextFiscalYearUsedinCalculatingBenefitCosts
Namespace Prefix: | pnw_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Defined Benefit Plan, Pre-65 Health Care Cost Trend Rate Assumed for Next Fiscal Year, Used in Calculating Benefit Obligation + References No definition available. + Details
Name: | pnw_DefinedBenefitPlanPre65HealthCareCostTrendRateAssumedforNextFiscalYearUsedinCalculatingBenefitObligation
Namespace Prefix: | pnw_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Defined Benefit Plan, Pre-65 Participant, Period after which Rate Reaches Ultimate Trend Rate, used in Calculating Benefit Costs + References No definition available. + Details
Name: | pnw_DefinedBenefitPlanPre65ParticipantPeriodafterwhichRateReachesUltimateTrendRateusedinCalculatingBenefitCosts
Namespace Prefix: | pnw_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Defined Benefit Plan, Pre-65 Participant, Period after which Rate Reaches Ultimate Trend Rate, used in Calculating Benefit Obligation + References No definition available. + Details
Name: | pnw_DefinedBenefitPlanPre65ParticipantPeriodafterwhichRateReachesUltimateTrendRateusedinCalculatingBenefitObligations
Namespace Prefix: | pnw_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Defined Benefit Plan, Ultimate Health Care Cost Trend Rate used in Calculating Benefit Obligations + References No definition available. + Details
Name: | pnw_DefinedBenefitPlanUltimateHealthCareCostTrendRateusedinCalculatingBenefitObligations
Namespace Prefix: | pnw_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedInCalculationsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointChangeInAssumedHealthCareCostTrendRatesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnAccumulatedPostretirementBenefitObligation1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of decrease in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point decrease in assumed health care cost trend rate. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnServiceAndInterestCostComponents1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnAccumulatedPostretirementBenefitObligation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of increase in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point increase in assumed health care cost trend rate. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnServiceAndInterestCostComponents
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Ultimate trend rate for health care cost for defined benefit postretirement plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate1
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanWeightedAverageAssumptionsUsedInCalculatingNetPeriodicBenefitCostAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |