Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - APS - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Aug. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Nuclear decommissioning trust fund assets
Fair Value		$ 1,211,655		$ 1,211,655		$ 1,087,235
Total Unrealized Gains		325,782		325,782		230,781
Total Unrealized Losses		(476)		(476)		(7,237)
Amortized cost		696,000		696,000		635,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains		4,736	$ 653	8,482	$ 2,952
Realized losses		(2,360)	(1,965)	(5,465)	(6,990)
Proceeds from the sale of securities		211,641	173,277	520,996	443,040
Fair value of fixed income securities, summarized by contractual maturities
Employee medical claims amount	$ 15,000
Equity securities
Nuclear decommissioning trust fund assets
Equity securities		487,449		487,449		447,138
Total Unrealized Gains		297,032		297,032		222,147
Total Unrealized Losses		0		0		(459)
Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value		724,110		724,110		637,356
Total Unrealized Gains		28,750		28,750		8,634
Total Unrealized Losses		(476)		(476)		(6,778)
Fair value of fixed income securities, summarized by contractual maturities
Less than one year		109,950		109,950
1 year – 5 years		300,311		300,311
5 years – 10 years		104,693		104,693
Greater than 10 years		209,156		209,156
Total		724,110		724,110
Other
Nuclear decommissioning trust fund assets
Fair Value		96		96		2,741
Total Unrealized Gains		0		0		0
Total Unrealized Losses		0		0		0
Nuclear Decommissioning Trusts [Member]
Nuclear decommissioning trust fund assets
Fair Value		967,673		967,673		851,134
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains		4,732	653	8,478	2,951
Realized losses		(2,360)	(1,965)	(5,465)	(6,990)
Proceeds from the sale of securities		155,386	148,150	371,538	401,396
Nuclear Decommissioning Trusts [Member] | Equity securities
Nuclear decommissioning trust fund assets
Equity securities		485,467		485,467		402,008
Nuclear Decommissioning Trusts [Member] | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value		483,528		483,528		446,978
Fair value of fixed income securities, summarized by contractual maturities
Less than one year		40,309		40,309
1 year – 5 years		133,488		133,488
5 years – 10 years		103,973		103,973
Greater than 10 years		205,758		205,758
Total		483,528		483,528
Nuclear Decommissioning Trusts [Member] | Other
Nuclear decommissioning trust fund assets
Fair Value		(1,322)		(1,322)		2,148
Other Special Use Funds [Member]
Nuclear decommissioning trust fund assets
Fair Value		243,982		243,982		236,101
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains		4	0	4	1
Realized losses		0	0	0	0
Proceeds from the sale of securities		56,255	$ 25,127	149,458	$ 41,644
Other Special Use Funds [Member] | Equity securities
Nuclear decommissioning trust fund assets
Equity securities		1,982		1,982		45,130
Other Special Use Funds [Member] | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value		240,582		240,582		190,378
Other Special Use Funds [Member] | Other
Nuclear decommissioning trust fund assets
Fair Value		1,418		1,418		$ 593
Coal Reclamation Escrow Accounts [Member] | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year		32,628		32,628
1 year – 5 years		25,928		25,928
5 years – 10 years		720		720
Greater than 10 years		3,398		3,398
Total		62,674		62,674
Active Union Medical Trust [Member] | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year		37,013		37,013
1 year – 5 years		140,895		140,895
5 years – 10 years		0		0
Greater than 10 years		0		0
Total		$ 177,908		$ 177,908