CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Current Assets:
Cash and cash equivalents	$ 122,768	$ 145,944
Accounts receivable less allowance for uncollectible accounts	437,492	355,091
Materials, supplies and fuel, at average cost	146,454	129,663
Current income taxes receivable	0	82
Deferred income taxes	134,426	104,958
Other current assets	35,263	36,782
Total Current Assets	876,403	772,520
Public Utilities, Property, Plant and Equipment, Net [Abstract]
Plant in service	12,025,938	11,923,717
Construction work-in-progress	617,591	487,427
Total	12,643,529	12,411,144
Less accumulated provision for depreciation	3,300,961	3,184,071
Public Utilities, Property, Plant and Equipment, Net	9,342,568	9,227,073
Investments and other property, net	57,634	57,021
Other Assets, Noncurrent [Abstract]
Deferred energy	95,599	102,525
Regulatory assets	1,161,651	1,186,127
Regulatory asset for pension plans	209,698	215,656
Other Assets, Noncurrent	79,230	74,206
Total Deferred Charges and Other Assets	1,546,178	1,578,514
Assets	11,822,783	11,635,128
Current Liabilities:
Current maturities of long-term debt	8,054	139,985
Accounts Payable	321,962	312,990
Accrued expenses	118,020	128,144
Deferred energy	245,126	245,164
Other current liabilities	72,034	65,572
Total Current Liabilities	765,196	891,855
Long-term debt	5,130,322	5,008,931
Commitments and Contingencies	0	0
Deferred Credits and Other Liabilities:
Deferred income taxes	1,367,358	1,306,510
Deferred investment tax credit	14,861	16,140
Accrued retirement benefits	97,258	92,351
Regulatory liabilities	521,731	486,259
Other deferred credits and liabilities	509,332	427,003
Total Deferred Credits and Other Liabilities	2,510,540	2,328,263
Shareholders' Equity
Common stock	236,000	236,000
Other paid-in capital	2,713,736	2,713,736
Retained earnings	475,089	464,277
Accumulated other comprehensive loss	(8,100)	(7,934)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	3,416,725	3,406,079
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	11,822,783	11,635,128
NPC [Member]
Current Assets:
Cash and cash equivalents	49,052	65,887
Accounts receivable less allowance for uncollectible accounts	334,136	233,096
Materials, supplies and fuel, at average cost	76,877	72,529
Deferred income taxes	111,269	88,782
Other current assets	25,782	28,943
Total Current Assets	597,116	489,237
Public Utilities, Property, Plant and Equipment, Net [Abstract]
Plant in service	8,380,532	8,345,771
Construction work-in-progress	482,282	352,541
Total	8,862,814	8,698,312
Less accumulated provision for depreciation	2,003,431	1,906,617
Public Utilities, Property, Plant and Equipment, Net	6,859,383	6,791,695
Investments and other property, net	51,092	50,768
Other Assets, Noncurrent [Abstract]
Deferred energy	95,599	102,525
Regulatory assets	845,591	852,989
Regulatory asset for pension plans	105,465	108,528
Other Assets, Noncurrent	58,182	46,855
Total Deferred Charges and Other Assets	1,104,837	1,110,897
Assets	8,612,428	8,442,597
Current Liabilities:
Current maturities of long-term debt	7,927	139,985
Accounts Payable	206,962	182,183
Accounts Payable, Related Parties, Current	39,657	28,429
Accrued expenses	81,663	89,311
Deferred energy	166,000	159,799
Other current liabilities	57,232	50,725
Total Current Liabilities	559,441	650,432
Long-term debt	3,441,672	3,319,605
Commitments and Contingencies	0	0
Deferred Credits and Other Liabilities:
Deferred income taxes	1,046,235	997,921
Deferred investment tax credit	5,389	6,098
Accrued retirement benefits	12,060	9,454
Regulatory liabilities	302,222	274,951
Other deferred credits and liabilities	414,324	335,159
Total Deferred Credits and Other Liabilities	1,780,230	1,623,583
Shareholders' Equity
Common stock	1	1
Other paid-in capital	2,308,219	2,308,219
Retained earnings	526,855	544,874
Accumulated other comprehensive loss	(3,990)	(4,117)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,831,085	2,848,977
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	8,612,428	8,442,597
SPPC [Member]
Current Assets:
Cash and cash equivalents	45,243	55,195
Accounts receivable less allowance for uncollectible accounts	103,119	121,863
Materials, supplies and fuel, at average cost	69,577	57,134
Intercompany income taxes receivable	10,351	10,351
Deferred income taxes	33,321	32,311
Other current assets	8,792	7,504
Total Current Assets	270,403	284,358
Public Utilities, Property, Plant and Equipment, Net [Abstract]
Plant in service	3,645,406	3,577,946
Construction work-in-progress	135,309	134,886
Total	3,780,715	3,712,832
Less accumulated provision for depreciation	1,297,530	1,277,454
Public Utilities, Property, Plant and Equipment, Net	2,483,185	2,435,378
Investments and other property, net	6,189	5,901
Other Assets, Noncurrent [Abstract]
Regulatory assets	316,060	333,138
Regulatory asset for pension plans	101,383	104,159
Other Assets, Noncurrent	15,271	21,074
Total Deferred Charges and Other Assets	432,714	458,371
Assets	3,192,491	3,184,008
Current Liabilities:
Current maturities of long-term debt	127	0
Accounts Payable	89,686	99,897
Accounts Payable, Related Parties, Current	20,275	27,788
Accrued expenses	30,645	32,840
Deferred energy	79,126	85,365
Other current liabilities	14,803	14,846
Total Current Liabilities	234,662	260,736
Long-term debt	1,178,650	1,179,326
Commitments and Contingencies	0	0
Deferred Credits and Other Liabilities:
Deferred income taxes	410,231	398,787
Deferred investment tax credit	9,472	10,042
Accrued retirement benefits	75,233	74,297
Regulatory liabilities	219,509	211,308
Other deferred credits and liabilities	78,784	74,970
Total Deferred Credits and Other Liabilities	793,229	769,404
Shareholders' Equity
Common stock	4	4
Other paid-in capital	1,111,262	1,111,262
Retained earnings	(124,017)	(135,340)
Accumulated other comprehensive loss	(1,299)	(1,384)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	985,950	974,542
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 3,192,491	$ 3,184,008