Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
	Jan. 04, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Fair value of fixed income securities, summarized by contractual maturities
Transfer to active union medical account	$ 106,000
APS
Nuclear decommissioning trust fund assets
Fair Value		$ 1,581,524		$ 1,581,524		$ 1,392,944
Total Unrealized Gains				541,701		468,247
Total Unrealized Losses				(1,357)		(398)
Amortized cost		815,000		815,000		687,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains		1,406	$ 4,500	4,374	$ 7,813
Realized losses		(1,146)	(1,621)	(5,294)	(3,848)
Proceeds from the sale of securities		207,864	196,772	587,842	391,859
Fair value of fixed income securities, summarized by contractual maturities
Employee medical claims amount						14,000	$ 15,000
APS | Equity securities
Nuclear decommissioning trust fund assets
Equity securities		745,489		745,489		677,188
Total Unrealized Gains				510,432		421,666
Total Unrealized Losses				0		0
APS | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value		844,589		844,589		733,080
Total Unrealized Gains				31,269		46,581
Total Unrealized Losses				(1,357)		(398)
Fair value of fixed income securities, summarized by contractual maturities
Less than one year		94,537		94,537
1 year – 5 years		331,662		331,662
5 years – 10 years		195,426		195,426
Greater than 10 years		222,964		222,964
Total		844,589		844,589
APS | Other
Nuclear decommissioning trust fund assets
Fair Value		(8,554)		(8,554)		(17,324)
Total Unrealized Gains				0		0
Total Unrealized Losses				0		0
Nuclear Decommissioning Trust | APS
Nuclear decommissioning trust fund assets
Fair Value		1,223,088		1,223,088		1,138,435
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains		1,406	4,500	4,374	7,813
Realized losses		(1,146)	(1,621)	(5,294)	(3,848)
Proceeds from the sale of securities		190,340	176,942	425,068	355,138
Nuclear Decommissioning Trust | APS | Equity securities
Nuclear decommissioning trust fund assets
Equity securities		725,585		725,585		639,851
Nuclear Decommissioning Trust | APS | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value		507,009		507,009		516,412
Fair value of fixed income securities, summarized by contractual maturities
Less than one year		28,151		28,151
1 year – 5 years		133,054		133,054
5 years – 10 years		131,462		131,462
Greater than 10 years		214,342		214,342
Total		507,009		507,009
Nuclear Decommissioning Trust | APS | Other
Nuclear decommissioning trust fund assets
Fair Value		(9,506)		(9,506)		(17,828)
Other Special Use Funds | APS
Nuclear decommissioning trust fund assets
Fair Value		358,436		358,436		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		0	0	0	0
Proceeds from the sale of securities		17,524	$ 19,830	162,774	$ 36,721
Other Special Use Funds | APS | Equity securities
Nuclear decommissioning trust fund assets
Equity securities		19,904		19,904		37,337
Other Special Use Funds | APS | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value		337,580		337,580		216,668
Other Special Use Funds | APS | Other
Nuclear decommissioning trust fund assets
Fair Value		952		952		$ 504
Coal Reclamation Escrow Account | APS | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year		26,123		26,123
1 year – 5 years		35,880		35,880
5 years – 10 years		2,676		2,676
Greater than 10 years		8,622		8,622
Total		73,301		73,301
Active union medical trust | APS | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year		40,263		40,263
1 year – 5 years		162,728		162,728
5 years – 10 years		61,288		61,288
Greater than 10 years		0		0
Total		$ 264,279		$ 264,279