Retirement Plans and Other Benefits - Fair Value of Pinnacle West's Pension Plan (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	$ 726,093	$ 903,085
Fair value of plan assets	2,733,476	3,057,027	$ 2,675,357
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	690,737	686,941
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	1,316,646	1,467,001
Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	90,955	99,910
Fair value of plan assets	723,677	1,022,371	$ 882,651
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	461,905	583,796
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	170,817	338,665
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	1,237,744	1,365,194
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	1,237,744	1,365,194
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	163,286	306,008
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	163,286	306,008
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	372,649	221,291
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	372,649	221,291
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	318,017	336,963
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	318,017	336,963
Other | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	78,902	100,599
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	78,902	100,599
Other | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	7,531	32,508
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	7,531	32,508
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	196,661	228,088
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	196,661	228,088
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	129,199	196,153
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	129,199	196,153
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	120,976	233,732
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	120,976	233,732
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	10,963	39,269
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	10,963	39,269
Equities | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	272,926	408,763
Fair value of plan assets	272,926	408,763
Equities | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	65,720	75,310
Fair value of plan assets	65,720	75,310
Real estate | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	165,123	171,569
Fair value of plan assets	165,123	171,569
Real estate | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	19,054	15,422
Fair value of plan assets	19,054	15,422
Fixed income securities | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	86,483	90,869
Fair value of plan assets	86,483	90,869
Partnerships | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Other	125,217	133,379
Fair value of plan assets	125,217	133,379
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	76,344	98,505
Fair value of plan assets	76,344	99,713
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	1,208
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	6,181	9,178
Fair value of plan assets	9,814	20,595
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	3,633
Other		11,268
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
Other		149
Defined Benefit Plan, Cash and Cash Equivalents [Member] | Pension Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	451	3,830
Defined Benefit Plan, Cash and Cash Equivalents [Member] | 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	451	3,830
Defined Benefit Plan, Cash and Cash Equivalents [Member] | Other Benefits
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category
Fair value of plan assets	93	143
Defined Benefit Plan, Cash and Cash Equivalents [Member] | 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	$ 93	$ 143