Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - APS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Nuclear decommissioning trust fund assets
Fair Value	$ 1,140,427		$ 1,140,427		$ 901,358
Total Unrealized Gains	291,752		291,752		260,160
Total Unrealized Losses	(11,470)		(11,470)		(2,996)
Amortized cost	667,000		667,000		467,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	653	$ 598	2,952	$ 3,921
Realized losses	(1,965)	(1,022)	(6,990)	(4,643)
Proceeds from the sale of securities	173,277	76,496	443,040	355,953
Equity securities
Nuclear decommissioning trust fund assets
Fair Value	478,035		478,035
Total Unrealized Gains	286,121		286,121
Total Unrealized Losses	(47)		(47)
Fair Value					425,044
Total Unrealized Gains					248,623
Total Unrealized Losses					0
Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	661,294		661,294		475,716
Total Unrealized Gains	5,631		5,631		11,537
Total Unrealized Losses	(11,423)		(11,423)		(2,996)
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	67,754		67,754
1 year – 5 years	258,003		258,003
5 years – 10 years	128,808		128,808
Greater than 10 years	206,729		206,729
Total	661,294		661,294
Other
Nuclear decommissioning trust fund assets
Fair Value	1,098		1,098		598
Total Unrealized Gains	0		0		0
Total Unrealized Losses	0		0		0
Nuclear Decommissioning Trusts
Nuclear decommissioning trust fund assets
Fair Value	906,687		906,687		871,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	653	598	2,951	3,904
Realized losses	(1,965)	(1,022)	(6,990)	(4,634)
Proceeds from the sale of securities	148,150	76,496	401,396	351,860
Nuclear Decommissioning Trusts | Equity securities
Nuclear decommissioning trust fund assets
Fair Value	466,002		466,002
Fair Value					424,614
Nuclear Decommissioning Trusts | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	441,309		441,309
Fair Value					446,277
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	19,917		19,917
1 year – 5 years	98,235		98,235
5 years – 10 years	126,279		126,279
Greater than 10 years	196,878		196,878
Total	441,309		441,309
Nuclear Decommissioning Trusts | Other
Nuclear decommissioning trust fund assets
Fair Value	(624)		(624)		109
Other Special Use Funds
Nuclear decommissioning trust fund assets
Fair Value	233,740		233,740		30,358
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	0	0	1	17
Realized losses	0	0	0	(9)
Proceeds from the sale of securities	25,127	$ 0	41,644	$ 4,093
Other Special Use Funds | Equity securities
Nuclear decommissioning trust fund assets
Fair Value	12,033		12,033
Fair Value					430
Other Special Use Funds | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	219,985		219,985
Fair Value					29,439
Other Special Use Funds | Other
Nuclear decommissioning trust fund assets
Fair Value	1,722		1,722		$ 489
Coal Reclamation Escrow Accounts | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	17,244		17,244
1 year – 5 years	17,170		17,170
5 years – 10 years	2,529		2,529
Greater than 10 years	9,851		9,851
Total	46,794		46,794
Active Union Medical Trust | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	30,593		30,593
1 year – 5 years	142,598		142,598
5 years – 10 years	0		0
Greater than 10 years	0		0
Total	$ 173,191		$ 173,191