CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
CURRENT ASSETS
Cash and cash equivalents	$ 63,139	$ 10,283
Customer and other receivables	258,874	266,426
Accrued unbilled revenues	93,434	128,165
Allowance for doubtful accounts	(8,366)	(8,171)
Materials and supplies (at average cost)	323,545	331,091
Fossil fuel (at average cost)	16,930	14,829
Income tax receivable	20,599	21,727
Assets from risk management activities (Note 7)	2,108	515
Deferred fuel and purchased power regulatory asset (Note 4)	77,730	70,137
Other regulatory assets (Note 4)	147,741	133,070
Other current assets	82,573	61,958
Total current assets	1,078,307	1,030,030
INVESTMENTS AND OTHER ASSETS
Nuclear decommissioning trust (Notes 11 and 12)	920,426	1,010,775
Other special use funds (Notes 11 and 12)	252,723	245,095
Other assets	97,822	96,953
Total investments and other assets	1,270,971	1,352,823
PROPERTY, PLANT AND EQUIPMENT
Plant in service and held for future use	19,930,983	19,836,292
Accumulated depreciation and amortization	(6,784,467)	(6,637,857)
Net	13,146,516	13,198,435
Construction work in progress	942,258	808,133
Intangible assets, net of accumulated amortization	279,238	290,564
Nuclear fuel, net of accumulated amortization	168,457	123,500
Total property, plant and equipment	14,637,407	14,522,538
DEFERRED DEBITS
Regulatory assets (Note 4)	1,302,448	1,304,073
Operating lease right-of-use assets	144,380	145,813
Assets for other postretirement benefits (Note 5)	96,243	90,570
Other	32,004	33,400
Total deferred debits	1,575,075	1,573,856
TOTAL ASSETS	18,561,760	18,479,247
CURRENT LIABILITIES
Current maturities of long-term debt (Note 3)	650,000	800,000
Accounts payable	301,325	346,448
Accrued taxes	194,732	144,899
Accrued interest	53,608	53,534
Common dividends payable	0	87,982
Short-term borrowings (Note 3)	563,000	114,675
Customer deposits	54,965	64,908
Liabilities from risk management activities (Note 7)	54,784	38,946
Liabilities for asset retirements	10,095	11,025
Operating lease liabilities	12,360	12,713
Regulatory liabilities (Note 4)	279,105	234,912
Other current liabilities	121,514	168,323
Total current liabilities	2,295,488	2,078,365
Long-term debt less current maturities (Note 3)	4,833,324	4,832,558
DEFERRED CREDITS AND OTHER
Deferred income taxes	2,016,770	1,992,339
Regulatory liabilities (Note 4)	2,067,801	2,267,835
Liabilities for asset retirements	649,226	646,193
Liabilities for pension benefits (Note 5)	273,284	280,185
Liabilities from risk management activities (Note 7)	32,577	33,186
Customer advances	212,545	215,330
Unrecorded Unconditional Purchase Obligation	166,796	165,695
Deferred investment tax credit	196,002	196,468
Unrecognized tax benefits	6,400	6,189
Operating lease liabilities	51,198	51,872
Other	163,517	159,844
Total deferred credits and other	5,836,116	6,015,136
COMMITMENTS AND CONTINGENCIES (SEE NOTE 8)
EQUITY
Common stock, no par value; authorized 150,000,000 shares, 112,563,610 and 112,540,126 issued at respective dates	2,664,387	2,659,561
Treasury stock at cost; 72,302 and 103,546 shares at respective dates	(7,000)	(9,427)
Total common stock	2,657,387	2,650,134
Retained earnings	2,867,610	2,837,610
Accumulated other comprehensive loss	(55,579)	(57,096)
Total shareholders’ equity	5,469,418	5,430,648
Noncontrolling interests (Note 6)	127,414	122,540
Total equity	5,596,832	5,553,188
TOTAL LIABILITIES AND EQUITY	18,561,760	18,479,247
APS
CURRENT ASSETS
Cash and cash equivalents	53,351	10,169
Customer and other receivables	258,457	255,479
Accrued unbilled revenues	93,434	128,165
Allowance for doubtful accounts	(8,366)	(8,171)
Materials and supplies (at average cost)	323,545	331,091
Fossil fuel (at average cost)	16,930	14,829
Income tax receivable	8,724	7,313
Assets from risk management activities (Note 7)	2,108	515
Deferred fuel and purchased power regulatory asset (Note 4)	77,730	70,137
Other regulatory assets (Note 4)	147,741	133,070
Other current assets	57,471	38,895
Total current assets	1,031,125	981,492
INVESTMENTS AND OTHER ASSETS
Nuclear decommissioning trust (Notes 11 and 12)	920,426	1,010,775
Other special use funds (Notes 11 and 12)	252,723	245,095
Other assets	44,681	43,781
Total investments and other assets	1,217,830	1,299,651
PROPERTY, PLANT AND EQUIPMENT
Plant in service and held for future use	19,927,522	19,832,805
Accumulated depreciation and amortization	(6,781,228)	(6,634,597)
Net	13,146,294	13,198,208
Construction work in progress	942,258	808,133
Intangible assets, net of accumulated amortization	279,082	290,409
Nuclear fuel, net of accumulated amortization	168,457	123,500
Total property, plant and equipment	14,637,029	14,522,156
DEFERRED DEBITS
Regulatory assets (Note 4)	1,302,448	1,304,073
Operating lease right-of-use assets	142,647	144,024
Assets for other postretirement benefits (Note 5)	92,391	86,736
Other	31,282	32,591
Total deferred debits	1,568,768	1,567,424
TOTAL ASSETS	18,454,752	18,370,723
CURRENT LIABILITIES
Current maturities of long-term debt (Note 3)	200,000	350,000
Accounts payable	294,037	338,006
Accrued taxes	190,571	136,328
Accrued interest	51,042	52,619
Common dividends payable	0	88,000
Short-term borrowings (Note 3)	430,000	0
Customer deposits	54,965	64,908
Liabilities from risk management activities (Note 7)	54,784	38,946
Liabilities for asset retirements	10,095	11,025
Operating lease liabilities	12,224	12,549
Regulatory liabilities (Note 4)	279,105	234,912
Other current liabilities	133,497	164,736
Total current liabilities	1,710,320	1,492,029
Long-term debt less current maturities (Note 3)	4,833,743	4,833,133
DEFERRED CREDITS AND OTHER
Deferred income taxes	2,057,824	2,033,096
Regulatory liabilities (Note 4)	2,067,801	2,267,835
Liabilities for asset retirements	649,226	646,193
Liabilities for pension benefits (Note 5)	255,749	262,243
Liabilities from risk management activities (Note 7)	32,577	33,186
Customer advances	212,545	215,330
Unrecorded Unconditional Purchase Obligation	166,796	165,695
Deferred investment tax credit	196,002	196,468
Unrecognized tax benefits	40,399	40,188
Operating lease liabilities	49,442	50,092
Other	141,984	136,432
Total deferred credits and other	5,870,345	6,046,758
COMMITMENTS AND CONTINGENCIES (SEE NOTE 8)
EQUITY
Common stock	178,162	178,162
Additional paid-in capital	2,721,696	2,721,696
Retained earnings	3,047,269	3,011,927
Accumulated other comprehensive loss	(34,197)	(35,522)
Total shareholders’ equity	5,912,930	5,876,263
Noncontrolling interests (Note 6)	127,414	122,540
Total equity	6,040,344	5,998,803
Total capitalization	10,874,087	10,831,936
TOTAL LIABILITIES AND EQUITY	18,454,752	18,370,723
Variable Interest Entity, Primary Beneficiary [Member]
PROPERTY, PLANT AND EQUIPMENT
Total property, plant and equipment	100,938	101,906
Variable Interest Entity, Primary Beneficiary [Member] | APS
PROPERTY, PLANT AND EQUIPMENT
Total property, plant and equipment	100,938	101,906
EQUITY
Noncontrolling interests (Note 6)	$ 127,414	$ 122,540