Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
APS
Nuclear decommissioning trust fund assets
Fair Value	$ 1,089,981		$ 1,089,981		$ 901,358
Total Unrealized Gains	261,238		261,238		260,160
Total Unrealized Losses	(9,805)		(9,805)		(2,996)
Amortized cost	639,000		639,000		467,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	1,484	$ 956	2,299	$ 3,323
Realized losses	(2,978)	(1,167)	(5,025)	(3,621)
Proceeds from the sale of securities	125,216	125,810	258,227	279,457
APS | Equity securities
Nuclear decommissioning trust fund assets
Fair Value	446,724		446,724
Total Unrealized Gains	254,342		254,342
Total Unrealized Losses	1		1
Fair Value					425,044
Total Unrealized Gains					248,623
Total Unrealized Losses					0
APS | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	641,400		641,400		475,716
Total Unrealized Gains	6,896		6,896		11,537
Total Unrealized Losses	(9,804)		(9,804)		(2,996)
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	48,856		48,856
1 year – 5 years	260,373		260,373
5 years – 10 years	130,314		130,314
Greater than 10 years	201,857		201,857
Total	641,400		641,400
APS | Other
Nuclear decommissioning trust fund assets
Fair Value	1,857		1,857		598
Total Unrealized Gains	0		0		0
Total Unrealized Losses	0		0		0
Nuclear Decommissioning Trusts | APS
Nuclear decommissioning trust fund assets
Fair Value	873,643		873,643		871,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	1,484	939	2,298	3,306
Realized losses	(2,978)	(1,159)	(5,025)	(3,612)
Proceeds from the sale of securities	122,790	124,238	253,246	275,364
Nuclear Decommissioning Trusts | APS | Equity securities
Nuclear decommissioning trust fund assets
Fair Value	432,615		432,615
Fair Value					424,614
Nuclear Decommissioning Trusts | APS | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	440,403		440,403
Fair Value					446,277
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	18,415		18,415
1 year – 5 years	105,470		105,470
5 years – 10 years	127,599		127,599
Greater than 10 years	188,919		188,919
Total	440,403		440,403
Nuclear Decommissioning Trusts | APS | Other
Nuclear decommissioning trust fund assets
Fair Value	625		625		109
Other Special Use Funds | APS
Nuclear decommissioning trust fund assets
Fair Value	216,338		216,338		30,358
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	0	17	1	17
Realized losses	0	(8)	0	(9)
Proceeds from the sale of securities	2,426	$ 1,572	4,981	$ 4,093
Other Special Use Funds | APS | Equity securities
Nuclear decommissioning trust fund assets
Fair Value	14,109		14,109
Fair Value					430
Other Special Use Funds | APS | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	200,997		200,997
Fair Value					29,439
Other Special Use Funds | APS | Other
Nuclear decommissioning trust fund assets
Fair Value	1,232		1,232		489
Coal Reclamation Escrow Accounts | APS | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	0		0
1 year – 5 years	11,706		11,706
5 years – 10 years	2,715		2,715
Greater than 10 years	12,938		12,938
Total	27,359		27,359
Coal Reclamation Escrow Accounts | 4CA
Nuclear decommissioning trust fund assets
Fair Value	2,000		2,000		$ 2,000
Active Union Medical Trust | APS | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	30,441		30,441
1 year – 5 years	143,197		143,197
5 years – 10 years	0		0
Greater than 10 years	0		0
Total	$ 173,638		$ 173,638