CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Current Assets:
Cash and cash equivalents	$ 100,335	$ 145,944
Accounts receivable less allowance for uncollectible accounts	325,388	355,091
Materials, supplies and fuel, at average cost	130,628	129,663
Current income taxes receivable	82	82
Deferred income taxes	117,079	104,958
Other current assets	49,802	36,782
Total Current Assets	723,314	772,520
Public Utilities, Property, Plant and Equipment, Net [Abstract]
Plant in service	11,955,428	11,923,717
Construction work-in-progress	556,170	487,427
Total	12,511,598	12,411,144
Less accumulated provision for depreciation	3,240,688	3,184,071
Public Utilities, Property, Plant and Equipment, Net	9,270,910	9,227,073
Investments and other property, net	59,273	57,021
Other Assets, Noncurrent [Abstract]
Deferred energy	99,566	102,525
Regulatory assets	1,174,069	1,186,127
Regulatory asset for pension plans	212,259	215,656
Other Assets, Noncurrent	88,020	74,206
Total Deferred Charges and Other Assets	1,573,914	1,578,514
Assets	11,627,411	11,635,128
Current Liabilities:
Current maturities of long-term debt	138,048	139,985
Accounts Payable	270,828	312,990
Accrued expenses	97,816	128,144
Deferred energy	233,071	245,164
Other current liabilities	68,489	65,572
Total Current Liabilities	808,252	891,855
Long-term debt	5,035,066	5,008,931
Deferred Credits and Other Liabilities:
Deferred income taxes	1,311,996	1,306,510
Deferred investment tax credit	15,500	16,140
Accrued retirement benefits	94,572	92,351
Regulatory liabilities	503,911	486,259
Other deferred credits and liabilities	470,633	427,003
Total Deferred Credits and Other Liabilities	2,396,612	2,328,263
Shareholders' Equity
Common stock	236,000	236,000
Other paid-in capital	2,713,736	2,713,736
Retained earnings	445,770	464,277
Accumulated other comprehensive loss	(8,025)	(7,934)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	3,387,481	3,406,079
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	11,627,411	11,635,128
NPC [Member]
Current Assets:
Cash and cash equivalents	38,710	65,887
Accounts receivable less allowance for uncollectible accounts	210,654	233,096
Materials, supplies and fuel, at average cost	74,829	72,529
Deferred income taxes	101,492	88,782
Other current assets	37,617	28,943
Total Current Assets	463,302	489,237
Public Utilities, Property, Plant and Equipment, Net [Abstract]
Plant in service	8,355,133	8,345,771
Construction work-in-progress	410,417	352,541
Total	8,765,550	8,698,312
Less accumulated provision for depreciation	1,950,742	1,906,617
Public Utilities, Property, Plant and Equipment, Net	6,814,808	6,791,695
Investments and other property, net	52,595	50,768
Other Assets, Noncurrent [Abstract]
Deferred energy	99,566	102,525
Regulatory assets	851,458	852,989
Regulatory asset for pension plans	106,877	108,528
Other Assets, Noncurrent	61,834	46,855
Total Deferred Charges and Other Assets	1,119,735	1,110,897
Assets	8,450,440	8,442,597
Current Liabilities:
Current maturities of long-term debt	137,929	139,985
Accounts Payable	161,525	182,183
Accounts Payable, Related Parties, Current	35,511	28,429
Accrued expenses	60,796	89,311
Deferred energy	160,890	159,799
Other current liabilities	54,744	50,725
Total Current Liabilities	611,395	650,432
Long-term debt	3,346,026	3,319,605
Deferred Credits and Other Liabilities:
Deferred income taxes	1,003,426	997,921
Deferred investment tax credit	5,743	6,098
Accrued retirement benefits	11,027	9,454
Regulatory liabilities	287,747	274,951
Other deferred credits and liabilities	376,352	335,159
Total Deferred Credits and Other Liabilities	1,684,295	1,623,583
Shareholders' Equity
Common stock	1	1
Other paid-in capital	2,308,219	2,308,219
Retained earnings	504,558	544,874
Accumulated other comprehensive loss	(4,054)	(4,117)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,808,724	2,848,977
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	8,450,440	8,442,597
SPPC [Member]
Current Assets:
Cash and cash equivalents	23,852	55,195
Accounts receivable less allowance for uncollectible accounts	114,499	121,863
Materials, supplies and fuel, at average cost	55,799	57,134
Intercompany income taxes receivable	10,351	10,351
Deferred income taxes	26,362	32,311
Other current assets	12,069	7,504
Total Current Assets	242,932	284,358
Public Utilities, Property, Plant and Equipment, Net [Abstract]
Plant in service	3,600,295	3,577,946
Construction work-in-progress	145,753	134,886
Total	3,746,048	3,712,832
Less accumulated provision for depreciation	1,289,946	1,277,454
Public Utilities, Property, Plant and Equipment, Net	2,456,102	2,435,378
Investments and other property, net	6,326	5,901
Other Assets, Noncurrent [Abstract]
Regulatory assets	322,611	333,138
Regulatory asset for pension plans	102,497	104,159
Other Assets, Noncurrent	19,890	21,074
Total Deferred Charges and Other Assets	444,998	458,371
Assets	3,150,358	3,184,008
Current Liabilities:
Current maturities of long-term debt	120	0
Accounts Payable	86,494	99,897
Accounts Payable, Related Parties, Current	18,506	27,788
Accrued expenses	27,566	32,840
Dividends Payable	0	0
Deferred energy	72,181	85,365
Other current liabilities	13,744	14,846
Total Current Liabilities	218,611	260,736
Long-term debt	1,179,041	1,179,326
Commitments and Contingencies	0	0
Deferred Credits and Other Liabilities:
Deferred income taxes	395,885	398,787
Deferred investment tax credit	9,757	10,042
Accrued retirement benefits	74,664	74,297
Regulatory liabilities	216,164	211,308
Other deferred credits and liabilities	83,008	74,970
Total Deferred Credits and Other Liabilities	779,478	769,404
Shareholders' Equity
Common stock	4	4
Other paid-in capital	1,111,262	1,111,262
Retained earnings	(136,696)	(135,340)
Accumulated other comprehensive loss	(1,342)	(1,384)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	973,228	974,542
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 3,150,358	$ 3,184,008