Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - APS - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Nuclear decommissioning trust fund assets
Fair Value	$ 1,277,365		$ 1,277,365		$ 1,255,870
Total Unrealized Gains	366,187		366,187		363,476
Total Unrealized Losses	(582)		(582)		(669)
Amortized cost	707,000		707,000		691,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	4,500	$ 2,643	7,813	$ 3,746
Realized losses	(1,621)	(1,700)	(3,848)	(3,105)
Proceeds from the sale of securities	196,772	130,306	391,859	309,354
Equity securities
Nuclear decommissioning trust fund assets
Equity securities	527,191		527,191		536,858
Total Unrealized Gains	316,494		316,494		337,681
Total Unrealized Losses	(58)		(58)		0
Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	755,778		755,778		716,137
Total Unrealized Gains	49,693		49,693		25,795
Total Unrealized Losses	(524)		(524)		(669)
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	99,934		99,934
1 year – 5 years	292,531		292,531
5 years – 10 years	127,711		127,711
Greater than 10 years	235,602		235,602
Total	755,778		755,778
Other
Nuclear decommissioning trust fund assets
Other	(5,604)		(5,604)		2,875
Total Unrealized Gains	0		0		0
Total Unrealized Losses	0		0		0
Nuclear Decommissioning Trust
Nuclear decommissioning trust fund assets
Fair Value	1,024,033		1,024,033		1,010,775
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	4,500	2,643	7,813	3,746
Realized losses	(1,621)	(1,700)	(3,848)	(3,105)
Proceeds from the sale of securities	176,942	93,559	355,138	216,152
Nuclear Decommissioning Trust | Equity securities
Nuclear decommissioning trust fund assets
Equity securities	518,236		518,236		529,716
Nuclear Decommissioning Trust | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	511,927		511,927		478,658
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	21,767		21,767
1 year – 5 years	137,754		137,754
5 years – 10 years	125,807		125,807
Greater than 10 years	226,599		226,599
Total	511,927		511,927
Nuclear Decommissioning Trust | Other
Nuclear decommissioning trust fund assets
Other	(6,130)		(6,130)		2,401
Other Special Use Funds
Nuclear decommissioning trust fund assets
Fair Value	253,332		253,332		245,095
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	19,830	$ 36,747	36,721	$ 93,202
Other Special Use Funds | Equity securities
Nuclear decommissioning trust fund assets
Equity securities	8,955		8,955		7,142
Other Special Use Funds | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	243,851		243,851		237,479
Other Special Use Funds | Other
Nuclear decommissioning trust fund assets
Other	526		526		$ 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	37,494		37,494
1 year – 5 years	11,371		11,371
5 years – 10 years	1,904		1,904
Greater than 10 years	9,003		9,003
Total	59,772		59,772
Active union medical trust | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	40,673		40,673
1 year – 5 years	143,406		143,406
5 years – 10 years	0		0
Greater than 10 years	0		0
Total	$ 184,079		$ 184,079