Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Arizona Public Service Company
Nuclear decommissioning trust fund assets
Fair Value	$ 1,077,239		$ 901,358
Unrealized Gains	250,485		260,160
Unrealized Losses	(8,606)		(2,996)
Available-for-sale securities, amortized cost basis	642,000		467,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	815	$ 2,367
Realized losses	(2,047)	(2,454)
Proceeds from the sale of securities	133,011	153,647
Arizona Public Service Company | Equity Securities
Nuclear decommissioning trust fund assets
Fair Value	434,906		425,044
Unrealized Gains	242,784		248,623
Unrealized Losses	(61)		0
Arizona Public Service Company | Fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	640,942		475,716
Unrealized Gains	7,701		11,537
Unrealized Losses	(8,545)		(2,996)
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	46,468
1 year – 5 years	260,798
5 years – 10 years	127,942
Greater than 10 years	205,734
Total	640,942
Arizona Public Service Company | Other
Nuclear decommissioning trust fund assets
Fair Value	1,391		598
Unrealized Gains	0		0
Unrealized Losses	0		0
Nuclear Decommissioning Trusts | Arizona Public Service Company
Nuclear decommissioning trust fund assets
Fair Value	861,439		871,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	814	2,367
Realized losses	(2,047)	(2,453)
Proceeds from the sale of securities	130,456
Nuclear Decommissioning Trusts | Arizona Public Service Company | Equity Securities
Nuclear decommissioning trust fund assets
Fair Value	420,265		424,614
Nuclear Decommissioning Trusts | Arizona Public Service Company | Fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	441,241		446,277
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Proceeds from the sale of securities		151,126
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	16,148
1 year – 5 years	109,466
5 years – 10 years	125,206
Greater than 10 years	190,421
Total	441,241
Nuclear Decommissioning Trusts | Arizona Public Service Company | Other
Nuclear decommissioning trust fund assets
Fair Value	(67)		109
Other Special Use Funds | Arizona Public Service Company
Nuclear decommissioning trust fund assets
Fair Value	215,800		30,358
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	1	0
Realized losses	0	(1)
Proceeds from the sale of securities	2,555
Other Special Use Funds | Arizona Public Service Company | Equity Securities
Nuclear decommissioning trust fund assets
Fair Value	14,641		430
Other Special Use Funds | Arizona Public Service Company | Fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	199,701		29,439
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Proceeds from the sale of securities		$ 2,521
Other Special Use Funds | Arizona Public Service Company | Other
Nuclear decommissioning trust fund assets
Fair Value	1,458		489
Coal Reclamation Escrow Accounts | Arizona Public Service Company | Fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	0
1 year – 5 years	7,541
5 years – 10 years	2,736
Greater than 10 years	15,313
Total	25,590
Coal Reclamation Escrow Accounts | 4CA
Nuclear decommissioning trust fund assets
Fair Value	2,000		$ 2,000
Active Union Medical Trust | Arizona Public Service Company | Fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	30,320
1 year – 5 years	143,791
5 years – 10 years	0
Greater than 10 years	0
Total	$ 174,111