Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - APS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Nuclear decommissioning trust fund assets
Fair Value	$ 1,157,516		$ 1,087,235
Total Unrealized Gains	289,123		230,781
Total Unrealized Losses	(2,254)		(7,237)
Amortized cost	670,000		635,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	1,103	$ 815
Realized losses	(1,405)	(2,047)
Proceeds from the sale of securities	179,048	133,011
Equity securities
Nuclear decommissioning trust fund assets
Fair Value	470,247
Total Unrealized Gains	273,817
Total Unrealized Losses	(4)
Fair Value			447,138
Total Unrealized Gains			222,147
Total Unrealized Losses			(459)
Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	683,076		637,356
Total Unrealized Gains	15,306		8,634
Total Unrealized Losses	(2,250)		(6,778)
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	86,284
1 year – 5 years	275,098
5 years – 10 years	123,923
Greater than 10 years	197,771
Total	683,076
Other
Nuclear decommissioning trust fund assets
Fair Value	4,193		2,741
Total Unrealized Gains	0		0
Total Unrealized Losses	0		0
Nuclear Decommissioning Trusts
Nuclear decommissioning trust fund assets
Fair Value	919,309		851,134
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	1,103	814
Realized losses	(1,405)	(2,047)
Proceeds from the sale of securities	122,593	130,456
Nuclear Decommissioning Trusts | Equity securities
Nuclear decommissioning trust fund assets
Fair Value	458,776
Fair Value			402,008
Nuclear Decommissioning Trusts | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	457,903
Fair Value			446,978
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	26,100
1 year – 5 years	120,511
5 years – 10 years	120,342
Greater than 10 years	190,950
Total	457,903
Nuclear Decommissioning Trusts | Other
Nuclear decommissioning trust fund assets
Fair Value	2,630		2,148
Other Special Use Funds
Nuclear decommissioning trust fund assets
Fair Value	238,207		236,101
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds
Realized gains	0	1
Realized losses	0	0
Proceeds from the sale of securities	56,455	$ 2,555
Other Special Use Funds | Equity securities
Nuclear decommissioning trust fund assets
Fair Value	11,471
Fair Value			45,130
Other Special Use Funds | Available for sale-fixed income securities
Nuclear decommissioning trust fund assets
Fair Value	225,173
Fair Value			190,378
Other Special Use Funds | Other
Nuclear decommissioning trust fund assets
Fair Value	1,563		$ 593
Coal Reclamation Escrow Accounts | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	20,085
1 year – 5 years	15,960
5 years – 10 years	3,581
Greater than 10 years	6,821
Total	46,447
Active Union Medical Trust | Available for sale-fixed income securities
Fair value of fixed income securities, summarized by contractual maturities
Less than one year	40,099
1 year – 5 years	138,627
5 years – 10 years	0
Greater than 10 years	0
Total	$ 178,726