CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Current Assets:
Cash and cash equivalents	$ 145,944	$ 86,189
Accounts receivable less allowance for uncollectible accounts	355,091	354,010
Materials, supplies and fuel, at average cost	129,663	114,520
Risk management assets (Note 9)	0	4,007
Current income taxes receivable	82	82
Deferred income taxes (Note 10)	104,958	130,800
Other current assets	36,782	42,330
Total Current Assets	772,520	731,938
Public Utilities, Property, Plant and Equipment, Net [Abstract]
Plant in service	11,923,717	11,068,518
Construction work-in-progress	487,427	908,579
Total (Note 1)	12,411,144	11,977,097
Less accumulated provision for depreciation	3,184,071	3,047,438
Public Utilities, Property, Plant and Equipment, Net	9,227,073	8,929,659
Investments and other property, net (Note 4)	57,021	61,613
Other Assets, Noncurrent [Abstract]
Deferred energy (Note 3)	102,525	117,623
Regulatory assets (Note 3)	1,186,127	1,237,159
Regulatory asset for pension plans (Note 3)	215,656	269,472
Other Assets, Noncurrent	74,206	166,882
Total Deferred Charges and Other Assets	1,578,514	1,791,136
Assets Held-for-sale, Long Lived	0	155,322
Assets	11,635,128	11,669,668
Current Liabilities:
Current maturities of long-term debt (Note 6)	139,985	355,929
Accounts Payable	312,990	346,409
Accrued expenses	128,144	133,851
Risk management liabilities (Note 9)	3,678	33,229
Deferred energy (Note 3)	245,164	315,839
Other current liabilities	61,894	70,638
Total Current Liabilities	891,855	1,255,895
Long-term debt (Note 6)	5,008,931	4,924,109
Commitments and Contingencies	0	0
Deferred Credits and Other Liabilities:
Deferred income taxes (Note 10)	1,306,510	1,246,410
Deferred investment tax credit	16,140	19,204
Accrued retirement benefits	92,351	148,841
Risk management liabilities (Note 9)	1,055	0
Regulatory liabilities (Note 3)	486,259	428,114
Other deferred credits and liabilities	425,948	265,571
Total Deferred Credits and Other Liabilities	2,328,263	2,108,140
Liabilities Held for Sale (Note 16)	0	30,706
Shareholders' Equity
Common stock	236,000	235,323
Other paid-in capital	2,713,736	2,705,954
Retained earnings	464,277	416,432
Accumulated other comprehensive loss	(7,934)	(6,891)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	3,406,079	3,350,818
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	11,635,128	11,669,668
NPC [Member]
Current Assets:
Cash and cash equivalents	65,887	60,077
Accounts receivable less allowance for uncollectible accounts	233,096	224,704
Materials, supplies and fuel, at average cost	72,529	66,459
Risk management assets (Note 9)	0	3,476
Intercompany income taxes receivable	0	0
Deferred income taxes (Note 10)	88,782	76,282
Other current assets	28,943	29,680
Total Current Assets	489,237	460,678
Public Utilities, Property, Plant and Equipment, Net [Abstract]
Plant in service	8,345,771	7,552,097
Construction work-in-progress	352,541	825,079
Total (Note 1)	8,698,312	8,377,176
Less accumulated provision for depreciation	1,906,617	1,828,366
Public Utilities, Property, Plant and Equipment, Net	6,791,695	6,548,810
Investments and other property, net (Note 4)	50,768	55,305
Other Assets, Noncurrent [Abstract]
Deferred energy (Note 3)	102,525	117,623
Regulatory assets (Note 3)	852,989	871,982
Regulatory asset for pension plans (Note 3)	108,528	133,410
Risk management assets (Note 9)	0	0
Other Assets, Noncurrent	46,855	114,016
Total Deferred Charges and Other Assets	1,110,897	1,237,031
Assets	8,442,597	8,301,824
Current Liabilities:
Current maturities of long-term debt (Note 6)	139,985	355,929
Accounts Payable	182,183	232,279
Accounts Payable, Related Parties, Current	28,429	29,334
Accrued expenses	89,311	89,638
Risk management liabilities (Note 9)	3,678	22,764
Deferred energy (Note 3)	159,799	171,349
Other current liabilities	47,047	54,607
Total Current Liabilities	650,432	955,900
Long-term debt (Note 6)	3,319,605	3,221,833
Deferred Credits and Other Liabilities:
Deferred income taxes (Note 10)	997,921	908,094
Deferred investment tax credit	6,098	7,255
Accrued retirement benefits	9,454	31,907
Risk management liabilities (Note 9)	0	0
Regulatory liabilities (Note 3)	274,951	225,983
Other deferred credits and liabilities	335,159	189,220
Total Deferred Credits and Other Liabilities	1,623,583	1,362,459
Shareholders' Equity
Common stock	1	1
Other paid-in capital	2,308,219	2,254,219
Retained earnings	544,874	511,288
Accumulated other comprehensive loss	(4,117)	(3,876)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,848,977	2,761,632
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	8,442,597	8,301,824
SPPC [Member]
Current Assets:
Cash and cash equivalents	55,195	9,552
Accounts receivable less allowance for uncollectible accounts	121,863	129,306
Materials, supplies and fuel, at average cost	57,134	48,061
Risk management assets (Note 9)	0	531
Intercompany income taxes receivable	10,351	10,351
Deferred income taxes (Note 10)	32,311	53,282
Other current assets	7,504	11,633
Total Current Assets	284,358	262,716
Public Utilities, Property, Plant and Equipment, Net [Abstract]
Plant in service	3,577,946	3,516,421
Construction work-in-progress	134,886	83,500
Total (Note 1)	3,712,832	3,599,921
Less accumulated provision for depreciation	1,277,454	1,219,072
Public Utilities, Property, Plant and Equipment, Net	2,435,378	2,380,849
Investments and other property, net (Note 4)	5,901	5,956
Other Assets, Noncurrent [Abstract]
Regulatory assets (Note 3)	333,138	365,177
Regulatory asset for pension plans (Note 3)	104,159	131,734
Risk management assets (Note 9)	0	0
Other Assets, Noncurrent	21,074	45,268
Total Deferred Charges and Other Assets	458,371	542,179
Assets Held-for-sale, Long Lived	0	155,322
Assets	3,184,008	3,347,022
Current Liabilities:
Current maturities of long-term debt (Note 6)	0	0
Accounts Payable	99,897	90,206
Accounts Payable, Related Parties, Current	27,788	10,812
Accrued expenses	32,840	33,788
Dividends Payable	0	54,000
Risk management liabilities (Note 9)	0	10,465
Deferred energy (Note 3)	85,365	144,490
Other current liabilities	14,846	16,029
Total Current Liabilities	260,736	359,790
Long-term debt (Note 6)	1,179,326	1,195,775
Deferred Credits and Other Liabilities:
Deferred income taxes (Note 10)	398,787	395,454
Deferred investment tax credit	10,042	11,949
Accrued retirement benefits	74,297	110,302
Risk management liabilities (Note 9)	0	0
Regulatory liabilities (Note 3)	211,308	202,131
Other deferred credits and liabilities	74,970	67,495
Total Deferred Credits and Other Liabilities	769,404	787,331
Liabilities Held for Sale (Note 16)	0	30,706
Shareholders' Equity
Common stock	4	4
Other paid-in capital	1,111,262	1,111,262
Retained earnings	(135,340)	(135,226)
Accumulated other comprehensive loss	(1,384)	(2,620)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	974,542	973,420
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 3,184,008	$ 3,347,022