Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - APS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Nuclear decommissioning trust fund assets
Fair Value	$ 1,600,328		$ 1,600,328		$ 1,392,944
Total Unrealized Gains			539,113		468,247
Total Unrealized Losses			(1,790)		(398)
Amortized cost	832,000		832,000		687,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	1,652	$ 2,933	6,026	$ 10,746
Realized losses	(1,562)	(765)	(6,856)	(4,613)
Proceeds from the sale of securities	209,336	216,026	797,178	607,885
Equity securities
Nuclear decommissioning trust fund assets
Equity securities	764,811		764,811		677,188
Total Unrealized Gains			511,895		421,666
Total Unrealized Losses			(27)		0
Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	857,668		857,668		733,080
Total Unrealized Gains			27,218		46,581
Total Unrealized Losses			(1,763)		(398)
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	93,172		93,172
1 year – 5 years	392,158		392,158
5 years – 10 years	150,861		150,861
Greater than 10 years	221,477		221,477
Total	857,668		857,668
Other
Nuclear decommissioning trust fund assets
Fair Value	(22,151)		(22,151)		(17,324)
Total Unrealized Gains			0		0
Total Unrealized Losses			0		0
Nuclear Decommissioning Trust
Nuclear decommissioning trust fund assets
Fair Value	1,225,485		1,225,485		1,138,435
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	1,652	2,933	6,026	10,746
Realized losses	(1,555)	(750)	(6,849)	(4,598)
Proceeds from the sale of securities	181,728	178,919	606,796	534,057
Nuclear Decommissioning Trust | Equity securities
Nuclear decommissioning trust fund assets
Equity securities	739,740		739,740		639,851
Nuclear Decommissioning Trust | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	509,288		509,288		516,412
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	22,350		22,350
1 year – 5 years	146,918		146,918
5 years – 10 years	127,053		127,053
Greater than 10 years	212,967		212,967
Total	509,288		509,288
Nuclear Decommissioning Trust | Other
Nuclear decommissioning trust fund assets
Fair Value	(23,543)		(23,543)		(17,828)
Other Special Use Funds
Nuclear decommissioning trust fund assets
Fair Value	374,843		374,843		254,509
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	0	0	0	0
Realized losses	(7)	(15)	(7)	(15)
Proceeds from the sale of securities	27,608	$ 37,107	190,382	$ 73,828
Other Special Use Funds | Equity securities
Nuclear decommissioning trust fund assets
Equity securities	25,071		25,071		37,337
Other Special Use Funds | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	348,380		348,380		216,668
Other Special Use Funds | Other
Nuclear decommissioning trust fund assets
Fair Value	1,392		1,392		$ 504
Coal Reclamation Escrow Account | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	30,762		30,762
1 year – 5 years	43,898		43,898
5 years – 10 years	1,797		1,797
Greater than 10 years	8,510		8,510
Total	84,967		84,967
Active Union Employee Medical Account | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	40,060		40,060
1 year – 5 years	201,342		201,342
5 years – 10 years	22,011		22,011
Greater than 10 years	0		0
Total	$ 263,413		$ 263,413