Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - APS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Nuclear decommissioning trust fund assets
Fair Value	$ 1,338,129		$ 1,338,129		$ 1,255,870
Total Unrealized Gains	406,270		406,270		363,476
Total Unrealized Losses	(710)		(710)		(669)
Amortized cost	726,000		726,000		691,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	2,933	$ 4,736	10,746	$ 8,482
Realized losses	(765)	(2,360)	(4,613)	(5,465)
Proceeds from the sale of securities	216,026	211,641	607,885	520,996
Equity securities
Nuclear decommissioning trust fund assets
Equity securities	578,472		578,472		536,858
Total Unrealized Gains	359,112		359,112		337,681
Total Unrealized Losses	(36)		(36)		0
Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	772,361		772,361		716,137
Total Unrealized Gains	47,158		47,158		25,795
Total Unrealized Losses	(674)		(674)		(669)
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	98,163		98,163
1 year – 5 years	318,864		318,864
5 years – 10 years	125,391		125,391
Greater than 10 years	229,943		229,943
Total	772,361		772,361
Other
Nuclear decommissioning trust fund assets
Other	(12,704)		(12,704)		2,875
Total Unrealized Gains	0		0		0
Total Unrealized Losses	0		0		0
Nuclear Decommissioning Trust
Nuclear decommissioning trust fund assets
Fair Value	1,069,837		1,069,837		1,010,775
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	2,933	4,732	10,746	8,478
Realized losses	(750)	(2,360)	(4,598)	(5,465)
Proceeds from the sale of securities	178,919	155,386	534,057	371,538
Nuclear Decommissioning Trust | Equity securities
Nuclear decommissioning trust fund assets
Equity securities	570,382		570,382		529,716
Nuclear Decommissioning Trust | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	513,603		513,603		478,658
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	23,079		23,079
1 year – 5 years	146,885		146,885
5 years – 10 years	122,619		122,619
Greater than 10 years	221,020		221,020
Total	513,603		513,603
Nuclear Decommissioning Trust | Other
Nuclear decommissioning trust fund assets
Other	(14,148)		(14,148)		2,401
Other Special Use Funds
Nuclear decommissioning trust fund assets
Fair Value	268,292		268,292		245,095
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	0	4	0	4
Realized losses	(15)	0	(15)	0
Proceeds from the sale of securities	37,107	$ 56,255	73,828	$ 149,458
Other Special Use Funds | Equity securities
Nuclear decommissioning trust fund assets
Equity securities	8,090		8,090		7,142
Other Special Use Funds | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	258,758		258,758		237,479
Other Special Use Funds | Other
Nuclear decommissioning trust fund assets
Other	1,444		1,444		$ 474
Coal Mine Reclamation Escrow Account | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	34,578		34,578
1 year – 5 years	29,041		29,041
5 years – 10 years	2,772		2,772
Greater than 10 years	8,923		8,923
Total	75,314		75,314
Active union medical trust | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	40,506		40,506
1 year – 5 years	142,938		142,938
5 years – 10 years	0		0
Greater than 10 years	0		0
Total	$ 183,444		$ 183,444