CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
CURRENT ASSETS
Cash and cash equivalents	$ 29,852	$ 5,766
Customer and other receivables	361,951	267,887
Accrued unbilled revenues	155,836	137,170
Allowance for doubtful accounts	(7,282)	(4,069)
Materials and supplies (at average cost)	293,899	269,065
Fossil fuel (at average cost)	18,527	25,029
Income tax receivable	14,063		$ 0
Assets from risk management activities (Note 7)	817	1,113
Deferred fuel and purchased power regulatory asset (Note 4)	59,474	37,164
Other regulatory assets (Note 4)	138,033	129,738
Other current assets	67,985	56,128
Total current assets	1,133,155	924,991
INVESTMENTS AND OTHER ASSETS
Nuclear decommissioning trust (Notes 11 and 12)	967,673	851,134
Other special use funds (Notes 11 and 12)	243,982	236,101
Other assets	102,116	103,247
Total investments and other assets	1,313,771	1,190,482
PROPERTY, PLANT AND EQUIPMENT
Plant in service and held for future use	19,677,773	18,736,628
Accumulated depreciation and amortization	(6,552,177)	(6,366,014)
Net	13,125,596	12,370,614
Construction work in progress	738,492	1,170,062
Palo Verde sale leaseback, net of accumulated depreciation (Note 6)	102,873	105,775
Intangible assets, net of accumulated amortization	266,587	262,902
Nuclear fuel, net of accumulated amortization	141,903	120,217
Total property, plant and equipment	14,375,451	14,029,570
DEFERRED DEBITS
Regulatory assets (Note 4)	1,329,446	1,342,941
Operating lease right-of-use assets (Note 16)	156,050	0
Assets for other postretirement benefits (Note 5)	31,717	46,906
Other	37,976	129,312
Total deferred debits	1,555,189	1,519,159
TOTAL ASSETS	18,377,566	17,664,202
CURRENT LIABILITIES
Accounts payable	276,117	277,336
Accrued taxes	220,930	154,819
Accrued interest	50,993	61,107
Common dividends payable	0	82,675
Short-term borrowings (Note 3)	57,375	76,400
Current maturities of long-term debt (Note 3)	450,000	500,000
Customer deposits	78,173	91,174
Liabilities from risk management activities (Note 7)	44,349	35,506
Liabilities for asset retirements	12,850	19,842
Operating lease liabilities (Note 16)	26,221	0
Regulatory liabilities (Note 4)	208,022	165,876
Other current liabilities	161,716	184,229
Total current liabilities	1,586,746	1,648,964
Long-term debt less current maturities (Note 3)	4,984,996	4,638,232
DEFERRED CREDITS AND OTHER
Deferred income taxes	1,975,989	1,807,421
Regulatory liabilities (Note 4)	2,310,131	2,325,976
Liabilities for asset retirements	736,079	706,703
Liabilities for pension benefits (Note 5)	297,843	443,170
Liabilities from risk management activities (Note 7)	27,305	24,531
Customer advances	192,374	137,153
Coal mine reclamation	165,695	212,785
Deferred investment tax credit	193,118	200,405
Unrecognized tax benefits	6,341	22,517
Operating lease liabilities (Note 16)	52,472	0
Other	166,772	147,640
Total deferred credits and other	6,124,119	6,028,301
COMMITMENTS AND CONTINGENCIES (SEE NOTE 8)
EQUITY
Common stock, no par value; authorized 150,000,000 shares, 112,403,751 and 112,159,896 issued at respective dates	2,654,430	2,634,265
Treasury stock at cost; 57,947 and 58,135 shares at respective dates	(5,117)	(4,825)
Total common stock	2,649,313	2,629,440
Retained earnings	2,949,891	2,641,183
Accumulated other comprehensive loss	(46,538)	(47,708)
Total shareholders’ equity	5,552,666	5,222,915
Noncontrolling interests (Note 6)	129,039	125,790
Total equity	5,681,705	5,348,705	5,135,730
TOTAL LIABILITIES AND EQUITY	18,377,566	17,664,202
APS
CURRENT ASSETS
Cash and cash equivalents	29,542	5,707
Customer and other receivables	351,029	257,654
Accrued unbilled revenues	155,836	137,170
Allowance for doubtful accounts	(7,282)	(4,069)
Materials and supplies (at average cost)	293,899	269,065
Fossil fuel (at average cost)	18,527	25,029
Income tax receivable	15,982	0
Assets from risk management activities (Note 7)	817	1,113
Deferred fuel and purchased power regulatory asset (Note 4)	59,474	37,164
Other regulatory assets (Note 4)	138,033	129,738
Other current assets	45,506	35,111
Total current assets	1,101,363	893,682
INVESTMENTS AND OTHER ASSETS
Nuclear decommissioning trust (Notes 11 and 12)	967,673	851,134
Other special use funds (Notes 11 and 12)	243,982	236,101
Other assets	55,846	40,817
Total investments and other assets	1,267,501	1,128,052
PROPERTY, PLANT AND EQUIPMENT
Plant in service and held for future use	19,674,286	18,733,142
Accumulated depreciation and amortization	(6,548,921)	(6,362,771)
Net	13,125,365	12,370,371
Construction work in progress	738,493	1,170,062
Palo Verde sale leaseback, net of accumulated depreciation (Note 6)	102,873	105,775
Intangible assets, net of accumulated amortization	266,432	262,746
Nuclear fuel, net of accumulated amortization	141,903	120,217
Total property, plant and equipment	14,375,066	14,029,171
DEFERRED DEBITS
Regulatory assets (Note 4)	1,329,446	1,342,941
Operating lease right-of-use assets (Note 16)	154,205	0
Assets for other postretirement benefits (Note 5)	28,071	43,212
Other	37,080	128,265
Total deferred debits	1,548,802	1,514,418
TOTAL ASSETS	18,292,732	17,565,323
CURRENT LIABILITIES
Accounts payable	268,163	266,277
Accrued taxes	215,320	176,357
Accrued interest	48,374	60,228
Common dividends payable	0	82,700
Short-term borrowings (Note 3)	2,900	0
Current maturities of long-term debt (Note 3)	450,000	500,000
Customer deposits	78,173	91,174
Liabilities from risk management activities (Note 7)	44,349	35,506
Liabilities for asset retirements	12,850	19,842
Operating lease liabilities (Note 16)	26,028	0
Regulatory liabilities (Note 4)	208,022	165,876
Other current liabilities	159,992	178,137
Total current liabilities	1,514,171	1,576,097
Long-term debt less current maturities (Note 3)	4,535,728	4,189,436
DEFERRED CREDITS AND OTHER
Deferred income taxes	1,976,662	1,812,664
Regulatory liabilities (Note 4)	2,310,131	2,325,976
Liabilities for asset retirements	736,079	706,703
Liabilities for pension benefits (Note 5)	281,605	425,404
Liabilities from risk management activities (Note 7)	27,305	24,531
Customer advances	192,374	137,153
Coal mine reclamation	165,695	212,785
Deferred investment tax credit	193,118	200,405
Unrecognized tax benefits	43,434	41,861
Operating lease liabilities (Note 16)	50,669	0
Other	143,190	125,511
Total deferred credits and other	6,120,262	6,012,993
COMMITMENTS AND CONTINGENCIES (SEE NOTE 8)
EQUITY
Total common stock	178,162	178,162
Additional paid-in capital	2,721,696	2,721,696
Retained earnings	3,119,977	2,788,256
Accumulated other comprehensive loss	(26,303)	(27,107)
Total shareholders’ equity	5,993,532	5,661,007
Noncontrolling interests (Note 6)	129,039	125,790
Total equity	6,122,571	5,786,797	$ 5,385,869
Total capitalization	10,658,299	9,976,233
TOTAL LIABILITIES AND EQUITY	$ 18,292,732	$ 17,565,323