Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - APS - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Nuclear decommissioning trust fund assets
Fair Value	$ 1,192,006		$ 1,192,006		$ 1,087,235
Total Unrealized Gains	315,933		315,933		230,781
Total Unrealized Losses	(696)		(696)		(7,237)
Amortized cost	672,000		672,000		635,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	2,643	$ 1,484	3,746	$ 2,299
Realized losses	(1,700)	(2,978)	(3,105)	(5,025)
Proceeds from the sale of securities	130,306	125,216	309,354	258,227
Equity securities
Nuclear decommissioning trust fund assets
Equity securities	487,894		487,894		447,138
Total Unrealized Gains	291,295		291,295		222,147
Total Unrealized Losses	0		0		459
Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	696,450		696,450		637,356
Total Unrealized Gains	24,638		24,638		8,634
Total Unrealized Losses	(696)		(696)		(6,778)
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	99,845		99,845
1 year – 5 years	284,763		284,763
5 years – 10 years	115,796		115,796
Greater than 10 years	196,046		196,046
Total	696,450		696,450
Other
Nuclear decommissioning trust fund assets
Fair Value	7,662		7,662		2,741
Total Unrealized Gains	0		0		0
Total Unrealized Losses	0		0		0
Nuclear Decommissioning Trust
Nuclear decommissioning trust fund assets
Fair Value	950,448		950,448		851,134
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	2,643	1,484	3,746	2,298
Realized losses	(1,700)	(2,978)	(3,105)	(5,025)
Proceeds from the sale of securities	93,559	122,790	216,152	253,246
Nuclear Decommissioning Trust | Equity securities
Nuclear decommissioning trust fund assets
Equity securities	474,694		474,694		402,008
Nuclear Decommissioning Trust | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	469,089		469,089		446,978
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	37,373		37,373
1 year – 5 years	126,745		126,745
5 years – 10 years	113,989		113,989
Greater than 10 years	190,982		190,982
Total	469,089		469,089
Nuclear Decommissioning Trust | Other
Nuclear decommissioning trust fund assets
Fair Value	6,665		6,665		2,148
Other Special Use Funds
Nuclear decommissioning trust fund assets
Fair Value	241,558		241,558		236,101
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	0	0	0	1
Realized losses	0	0	0	0
Proceeds from the sale of securities	36,747	$ 2,426	93,202	$ 4,981
Other Special Use Funds | Equity securities
Nuclear decommissioning trust fund assets
Equity securities	13,200		13,200		45,130
Other Special Use Funds | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	227,361		227,361		190,378
Other Special Use Funds | Other
Nuclear decommissioning trust fund assets
Fair Value	997		997		$ 593
Coal Reclamation Escrow Account | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	22,253		22,253
1 year – 5 years	17,449		17,449
5 years – 10 years	1,807		1,807
Greater than 10 years	5,064		5,064
Total	46,573		46,573
Active Union Medical Trust | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	40,219		40,219
1 year – 5 years	140,569		140,569
5 years – 10 years	0		0
Greater than 10 years	0		0
Total	$ 180,788		$ 180,788