Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
	Jan. 04, 2021	Mar. 31, 2021	Mar. 31, 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		$ 1,517,205		$ 1,392,944
Total Unrealized Gains		484,780		468,247
Total Unrealized Losses		(3,203)		(398)
Amortized cost		818,000		687,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains		2,968	$ 3,313
Realized losses		(4,148)	(2,227)
Proceeds from the sale of securities		379,978	195,087
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		689,570		677,188
Total Unrealized Gains		457,442		421,666
Total Unrealized Losses		0		0
APS | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value		841,772		733,080
Total Unrealized Gains		27,338		46,581
Total Unrealized Losses		(3,203)		(398)
Fair value of fixed income securities, summarized by contractual maturities
Less than one year		91,776
1 year – 5 years		342,607
5 years – 10 years		203,664
Greater than 10 years		203,725
Total		841,772
APS | Other
Nuclear decommissioning trust fund assets
Other		(14,137)		(17,324)
Total Unrealized Gains		0		0
Total Unrealized Losses		0		0
Nuclear Decommissioning Trust | APS
Nuclear decommissioning trust fund assets
Fair Value		1,159,699		1,138,435
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains		2,968	3,313
Realized losses		(4,148)	(2,227)
Proceeds from the sale of securities		234,728	178,196
Nuclear Decommissioning Trust | APS | Equity securities
Nuclear decommissioning trust fund assets
Equity securities		670,359		639,851
Nuclear Decommissioning Trust | APS | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value		504,878		516,412
Fair value of fixed income securities, summarized by contractual maturities
Less than one year		25,048
1 year – 5 years		147,347
5 years – 10 years		137,479
Greater than 10 years		195,004
Total		504,878
Nuclear Decommissioning Trust | APS | Other
Nuclear decommissioning trust fund assets
Other		(15,538)		(17,828)
Other Special Use Funds | APS
Nuclear decommissioning trust fund assets
Fair Value		357,506		254,509
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains		0	0
Realized losses		0	0
Proceeds from the sale of securities		145,250	$ 16,891
Other Special Use Funds | APS | Equity securities
Nuclear decommissioning trust fund assets
Equity securities		19,211		37,337
Other Special Use Funds | APS | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value		336,894		216,668
Other Special Use Funds | APS | Other
Nuclear decommissioning trust fund assets
Other		1,401		$ 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		26,259
1 year – 5 years		34,936
5 years – 10 years		2,708
Greater than 10 years		8,721
Total		72,624
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		40,469
1 year – 5 years		160,324
5 years – 10 years		63,477
Greater than 10 years		0
Total		$ 264,270