Retirement Plans and Other Postretirement Benefits - Fair Value of Pinnacle West's Pension Plan (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	$ 691,233	$ 832,143
Fair value of plan assets	3,886,544	3,318,351	$ 2,733,476
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	1,398,305	852,239
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	1,797,006	1,633,969
Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	118,440	107,473
Fair value of plan assets	961,165	837,494	$ 723,677
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	612,921	519,382
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	229,804	210,639
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	9,911	9,370
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	9,911	9,370
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	1,909	2,184
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	1,909	2,184
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	1,684,782	1,541,729
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	1,684,782	1,541,729
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	221,488	202,640
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	221,488	202,640
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	794,571	406,112
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	794,571	406,112
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	258,102	353,650
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	258,102	353,650
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	112,224	92,240
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	112,224	92,240
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	8,316	7,999
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	8,316	7,999
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	331,058	250,829
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	331,058	250,829
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	175,605	146,316
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	175,605	146,316
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	262,765	185,928
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	262,765	185,928
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	34,310	14,351
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	34,310	14,351
Equities | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	407,522	392,403
Fair value of plan assets	407,522	392,403
Equities | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	94,674	83,648
Fair value of plan assets	94,674	83,648
Real estate | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	191,595	171,645
Fair value of plan assets	191,595	171,645
Real estate | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	19,778	19,806
Fair value of plan assets	19,778	19,806
Fixed income | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other		98,065
Fair value of plan assets		98,065
Partnerships | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	22,420	103,796
Fair value of plan assets	22,420	103,796
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	69,696	66,234
Fair value of plan assets	69,696	66,234
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	3,988	4,019
Fair value of plan assets	146,983	6,900
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	142,995	2,881
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