Retirement Plans and Other Benefits - Fair Value of Pinnacle West's Pension Plan (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	$ 832,143	$ 726,093
Fair value of plan assets	3,318,351	2,733,476	$ 3,057,027
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	852,239	690,737
Pension Benefits | Significant Other Observable Inputs (Level 2)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	1,633,969	1,316,646
Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	107,473	90,955
Fair value of plan assets	837,494	723,677	$ 1,022,371
Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	519,382	461,905
Other Benefits | Significant Other Observable Inputs (Level 2)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	210,639	170,817
Cash and cash equivalents | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	9,370	451
Cash and cash equivalents | Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	9,370	451
Cash and cash equivalents | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	2,184	93
Cash and cash equivalents | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	2,184	93
Corporate | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	1,541,729	1,237,744
Corporate | Pension Benefits | Significant Other Observable Inputs (Level 2)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	1,541,729	1,237,744
Corporate | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	202,640	163,286
Corporate | Other Benefits | Significant Other Observable Inputs (Level 2)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	202,640	163,286
U.S. Treasury | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	406,112	372,649
U.S. Treasury | Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	406,112	372,649
U.S. Treasury | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	353,650	318,017
U.S. Treasury | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	353,650	318,017
Other fixed income | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	92,240	78,902
Other fixed income | Pension Benefits | Significant Other Observable Inputs (Level 2)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	92,240	78,902
Other fixed income | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	7,999	7,531
Other fixed income | Other Benefits | Significant Other Observable Inputs (Level 2)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	7,999	7,531
Common stock equities | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	250,829	196,661
Common stock equities | Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	250,829	196,661
Common stock equities | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	146,316	129,199
Common stock equities | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	146,316	129,199
Mutual funds | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	185,928	120,976
Mutual funds | Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	185,928	120,976
Mutual funds | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	14,351	10,963
Mutual funds | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	14,351	10,963
Equities | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	392,403	272,926
Fair value of plan assets	392,403	272,926
Equities | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	83,648	65,720
Fair value of plan assets	83,648	65,720
Real estate | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	171,645	165,123
Fair value of plan assets	171,645	165,123
Real estate | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	19,806	19,054
Fair value of plan assets	19,806	19,054
Fixed income securities | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	98,065	86,483
Fair value of plan assets	98,065	86,483
Partnerships | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	103,796	125,217
Fair value of plan assets	103,796	125,217
Short-term investments and other | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	66,234	76,344
Fair value of plan assets	66,234	76,344
Short-term investments and other | Pension Benefits | Significant Other Observable Inputs (Level 2)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	0
Short-term investments and other | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	4,019	6,181
Fair value of plan assets	6,900	9,814
Short-term investments and other | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	2,881	3,633
Short-term investments and other | Other Benefits | Significant Other Observable Inputs (Level 2)
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Gross fair value of plan assets	$ 0	$ 0