REGULATORY ACTIONS, Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Regulatory assets
Remaining Amortization Period	- - - -
Receiving Regulatory Treatment - Earning a Return	$ 357,058	[1]
Receiving Regulatory Treatment - Not Earning a Return	900,239
Pending Regulatory Treatment	144,486
Total regulatory assets	1,401,783		1,506,631
Regulatory liabilities
Remaining Amortization Period	Various
Receiving Regulatory Treatment - Earning a Return	433,706	[1]
Receiving Regulatory Treatment - Not Earning a Return	17,433
Pending Regulatory Treatment	35,120
Total regulatory liabilities	486,259		428,114
Cost of Removal [Member]
Regulatory liabilities
Remaining Amortization Period	Various
Receiving Regulatory Treatment - Earning a Return	422,033	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment
Total regulatory liabilities	422,033		382,634
Income Taxes Regulatory Liabilty [Member]
Regulatory liabilities
Receiving Regulatory Treatment - Earning a Return		[1]
Pending Regulatory Treatment
Total regulatory liabilities	17,433		19,506
Gain on Property Sales [Member]
Regulatory liabilities
Remaining Amortization Period	2013
Receiving Regulatory Treatment - Earning a Return	4,444	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment	32,844	[2]
Total regulatory liabilities	37,288		7,151
Renewable Energy Programs Regulatory Liability [Member]
Regulatory liabilities
Remaining Amortization Period	2012
Receiving Regulatory Treatment - Earning a Return	1,046	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment	0
Total regulatory liabilities	1,046		10,234
Other Regulatory Liabilities [Member]
Regulatory liabilities
Remaining Amortization Period	Various thru 2017
Receiving Regulatory Treatment - Earning a Return	6,183	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment	2,276
Total regulatory liabilities	8,459		8,589
Loss on Reacquired Debt [Member]
Regulatory assets
Remaining Amortization Period	Term of Related Debt
Receiving Regulatory Treatment - Earning a Return	72,408	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment
Total regulatory assets	72,408		84,692
Income Taxes Regulatory Asset [Member]
Regulatory assets
Remaining Amortization Period	Various
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return	251,314
Pending Regulatory Treatment
Total regulatory assets	251,314		257,078
Merger Costs [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2046
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return	268,668
Pending Regulatory Treatment
Total regulatory assets	268,668		282,535
Lenzie Generating Station [Member]
Regulatory assets
Remaining Amortization Period	2042
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return	67,351
Pending Regulatory Treatment
Total regulatory assets	67,351		77,524
Mohave Generating Station and Deferred Costs [Member]
Regulatory assets
Remaining Amortization Period	2017
Receiving Regulatory Treatment - Earning a Return	9,861	[1]
Receiving Regulatory Treatment - Not Earning a Return	12,654
Pending Regulatory Treatment	1,645	[2]
Total regulatory assets	24,160		25,849
Pinon Pine [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2029
Receiving Regulatory Treatment - Earning a Return	27,377	[1]
Receiving Regulatory Treatment - Not Earning a Return	7,016
Pending Regulatory Treatment
Total regulatory assets	34,393		38,960
Asset Retirement Obligation Costs [Member]
Regulatory assets
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment	67,891	[2]
Total regulatory assets	67,891		55,182
Conservation programs [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2017
Receiving Regulatory Treatment - Earning a Return	151,035	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment	7,412	[2]
Total regulatory assets	158,447		177,515
Energy Efficiency Program Costs [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2013
Receiving Regulatory Treatment - Earning a Return	30,379	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment
Total regulatory assets	30,379		30,409
Ely Energy Center [Member]
Regulatory assets
Remaining Amortization Period	2017
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return	23,403
Pending Regulatory Treatment	34,563	[2]
Total regulatory assets	57,966
Legacy Meters [Member]
Regulatory assets
Remaining Amortization Period	2013
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment	21,777	[2]
Total regulatory assets	21,777
Renewable Energy Programs Regulatory Asset [Member]
Regulatory assets
Receiving Regulatory Treatment - Earning a Return	29,592	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment
Total regulatory assets	29,592		2,627
Peabody Coal Costs [Member]
Regulatory assets
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return	17,899
Pending Regulatory Treatment
Total regulatory assets	17,899		17,738
Deferred Rate Increase [Member]
Regulatory assets
Remaining Amortization Period	2011
Receiving Regulatory Treatment - Earning a Return	12,177	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment
Total regulatory assets	12,177		91,678
Risk Management [Member]
Regulatory assets
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return	2,426
Pending Regulatory Treatment
Total regulatory assets	2,426		30,726
Other Costs [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2031
Receiving Regulatory Treatment - Earning a Return	24,229	[1]
Receiving Regulatory Treatment - Not Earning a Return	33,852
Pending Regulatory Treatment	11,198	[2]
Total regulatory assets	69,279		64,646
Subtotal - excluding pensions [Member]
Regulatory assets
Receiving Regulatory Treatment - Earning a Return	357,058	[1]
Receiving Regulatory Treatment - Not Earning a Return	684,583
Pending Regulatory Treatment	144,486
Pensions [Member]
Regulatory assets
Receiving Regulatory Treatment - Earning a Return	0	[1]
Receiving Regulatory Treatment - Not Earning a Return	215,656
Pending Regulatory Treatment	0
NPC [Member]
Regulatory assets
Receiving Regulatory Treatment - Earning a Return	241,149	[1]
Receiving Regulatory Treatment - Not Earning a Return	600,305
Pending Regulatory Treatment	120,063
Total regulatory assets	961,517		1,005,392
Regulatory liabilities
Receiving Regulatory Treatment - Earning a Return	234,064	[1]
Receiving Regulatory Treatment - Not Earning a Return	5,798
Pending Regulatory Treatment	35,089
Total regulatory liabilities	274,951		225,983
NPC [Member] | Cost of Removal [Member]
Regulatory liabilities
Remaining Amortization Period	Various
Receiving Regulatory Treatment - Earning a Return	232,093	[1]
Receiving Regulatory Treatment - Not Earning a Return	0
Pending Regulatory Treatment
Total regulatory liabilities	232,093		208,795
NPC [Member] | Income Taxes Regulatory Liabilty [Member]
Regulatory liabilities
Remaining Amortization Period	Various
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return	5,798
Pending Regulatory Treatment
Total regulatory liabilities	5,798		6,557
NPC [Member] | Gain on Property Sales [Member]
Regulatory liabilities
Remaining Amortization Period	-
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment	32,844	[3]
Total regulatory liabilities	32,844		0
NPC [Member] | Renewable Energy Programs Regulatory Liability [Member]
Regulatory liabilities
Remaining Amortization Period	2013
Receiving Regulatory Treatment - Earning a Return	1,046	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment
Total regulatory liabilities	1,046		7,797
NPC [Member] | Other Regulatory Liabilities [Member]
Regulatory liabilities
Remaining Amortization Period	2017
Receiving Regulatory Treatment - Earning a Return	925	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment	2,245
Total regulatory liabilities	3,170		2,834
NPC [Member] | Loss on Reacquired Debt [Member]
Regulatory assets
Remaining Amortization Period	Term of Related Debt
Receiving Regulatory Treatment - Earning a Return	39,958	[1]
Receiving Regulatory Treatment - Not Earning a Return	0
Pending Regulatory Treatment
Total regulatory assets	39,958		43,765
NPC [Member] | Income Taxes Regulatory Asset [Member]
Regulatory assets
Remaining Amortization Period	Various
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return	178,060
Pending Regulatory Treatment
Total regulatory assets	178,060		174,022
NPC [Member] | Merger Costs [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2044
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return	168,212
Pending Regulatory Treatment
Total regulatory assets	168,212		176,974
NPC [Member] | Lenzie Generating Station [Member]
Regulatory assets
Remaining Amortization Period	2042
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return	67,351
Pending Regulatory Treatment
Total regulatory assets	67,351		77,524
NPC [Member] | Mohave Generating Station and Deferred Costs [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2017
Receiving Regulatory Treatment - Earning a Return	9,861	[1]
Receiving Regulatory Treatment - Not Earning a Return	12,654
Pending Regulatory Treatment	1,645	[2]
Total regulatory assets	24,160		25,849
NPC [Member] | Asset Retirement Obligation Costs [Member]
Regulatory assets
Remaining Amortization Period	-
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment	60,797	[2]
Total regulatory assets	60,797		48,970
NPC [Member] | Conservation programs [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2017
Receiving Regulatory Treatment - Earning a Return	129,885	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment	4,004	[3]
Total regulatory assets	133,889		144,107
NPC [Member] | Energy Efficiency Program Costs [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2013
Receiving Regulatory Treatment - Earning a Return	25,250	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment
Total regulatory assets	25,250		24,905
NPC [Member] | Ely Energy Center [Member]
Regulatory assets
Remaining Amortization Period	2017
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return	23,403
Pending Regulatory Treatment	22,970	[2]
Total regulatory assets	46,373
NPC [Member] | Legacy Meters [Member]
Regulatory assets
Remaining Amortization Period	-
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment	21,777	[2]
Total regulatory assets	21,777
NPC [Member] | Renewable Energy Programs Regulatory Asset [Member]
Regulatory assets
Remaining Amortization Period	2013
Receiving Regulatory Treatment - Earning a Return	10,694	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment
Total regulatory assets	10,694
NPC [Member] | Peabody Coal Costs [Member]
Regulatory assets
Remaining Amortization Period	-
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return	17,899
Pending Regulatory Treatment
Total regulatory assets	17,899		17,738
NPC [Member] | Deferred Rate Increase [Member]
Regulatory assets
Remaining Amortization Period	2011
Receiving Regulatory Treatment - Earning a Return	12,177	[1]
Receiving Regulatory Treatment - Not Earning a Return
Pending Regulatory Treatment
Total regulatory assets	12,177		91,678
NPC [Member] | Risk Management [Member]
Regulatory assets
Remaining Amortization Period	-
Receiving Regulatory Treatment - Earning a Return		[1]
Receiving Regulatory Treatment - Not Earning a Return	2,426
Pending Regulatory Treatment
Total regulatory assets	2,426		20,261
NPC [Member] | Other Costs [Member]
Regulatory assets
Remaining Amortization Period	2017
Receiving Regulatory Treatment - Earning a Return	13,324	[1]
Receiving Regulatory Treatment - Not Earning a Return	21,772
Pending Regulatory Treatment	8,870	[2],[3]
Total regulatory assets	43,966		26,189
NPC [Member] | Subtotal - excluding pensions [Member]
Regulatory assets
Receiving Regulatory Treatment - Earning a Return	241,149	[1]
Receiving Regulatory Treatment - Not Earning a Return	491,777
Pending Regulatory Treatment	120,063
NPC [Member] | Pensions [Member]
Regulatory assets
Receiving Regulatory Treatment - Earning a Return	0	[1]
Receiving Regulatory Treatment - Not Earning a Return	108,528
Pending Regulatory Treatment	0
SPPC [Member]
Regulatory assets
Receiving Regulatory Treatment - Earning a Return	115,909	[1]
Receiving Regulatory Treatment - Not Earning a Return	296,965
Pending Regulatory Treatment	24,423
Total regulatory assets	437,297		496,911
Regulatory liabilities
Receiving Regulatory Treatment - Earning a Return	199,642	[1]
Receiving Regulatory Treatment - Not Earning a Return	11,635
Pending Regulatory Treatment	31
Total regulatory liabilities	211,308		202,131
SPPC [Member] | Cost of Removal [Member]
Regulatory liabilities
Remaining Amortization Period	Various
Receiving Regulatory Treatment - Earning a Return	189,940	[1]
Receiving Regulatory Treatment - Not Earning a Return	0
Pending Regulatory Treatment	0
Total regulatory liabilities	189,940		173,839
SPPC [Member] | Income Taxes Regulatory Liabilty [Member]
Regulatory liabilities
Remaining Amortization Period	Various
Receiving Regulatory Treatment - Earning a Return	0	[1]
Receiving Regulatory Treatment - Not Earning a Return	11,635
Pending Regulatory Treatment	0
Total regulatory liabilities	11,635		12,949
SPPC [Member] | Gain on Property Sales [Member]
Regulatory liabilities
Remaining Amortization Period	2013
Receiving Regulatory Treatment - Earning a Return	4,444	[1]
Receiving Regulatory Treatment - Not Earning a Return	0
Pending Regulatory Treatment	0
Total regulatory liabilities	4,444		7,151
SPPC [Member] | Renewable Energy Programs Regulatory Liability [Member]
Regulatory liabilities
Remaining Amortization Period	-
Receiving Regulatory Treatment - Earning a Return	0	[1]
Receiving Regulatory Treatment - Not Earning a Return	0
Pending Regulatory Treatment	0
Total regulatory liabilities	0		2,437
SPPC [Member] | Other Regulatory Liabilities [Member]
Regulatory liabilities
Remaining Amortization Period	Various thru 2043
Receiving Regulatory Treatment - Earning a Return	5,258	[1]
Receiving Regulatory Treatment - Not Earning a Return	0
Pending Regulatory Treatment	31	[3]
Total regulatory liabilities	5,289		5,755
SPPC [Member] | Loss on Reacquired Debt [Member]
Regulatory assets
Remaining Amortization Period	Term of Related Debt
Receiving Regulatory Treatment - Earning a Return	32,450	[1]
Receiving Regulatory Treatment - Not Earning a Return	0
Pending Regulatory Treatment	0
Total regulatory assets	32,450		40,927
SPPC [Member] | Income Taxes Regulatory Asset [Member]
Regulatory assets
Remaining Amortization Period	Various
Receiving Regulatory Treatment - Earning a Return	0	[1]
Receiving Regulatory Treatment - Not Earning a Return	73,254
Pending Regulatory Treatment	0
Total regulatory assets	73,254		83,056
SPPC [Member] | Merger Costs [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2046
Receiving Regulatory Treatment - Earning a Return	0	[1]
Receiving Regulatory Treatment - Not Earning a Return	100,456
Pending Regulatory Treatment	0
Total regulatory assets	100,456		105,561
SPPC [Member] | Pinon Pine [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2029
Receiving Regulatory Treatment - Earning a Return	27,377	[1]
Receiving Regulatory Treatment - Not Earning a Return	7,016
Pending Regulatory Treatment	0
Total regulatory assets	34,393		38,960
SPPC [Member] | Asset Retirement Obligation Costs [Member]
Regulatory assets
Remaining Amortization Period	-
Receiving Regulatory Treatment - Earning a Return	0	[1]
Receiving Regulatory Treatment - Not Earning a Return	0
Pending Regulatory Treatment	7,094	[2]
Total regulatory assets	7,094		6,212
SPPC [Member] | Conservation programs [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2013
Receiving Regulatory Treatment - Earning a Return	21,150	[1]
Receiving Regulatory Treatment - Not Earning a Return	0
Pending Regulatory Treatment	3,408	[3]
Total regulatory assets	24,558		33,408
SPPC [Member] | Energy Efficiency Program Costs [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2013
Receiving Regulatory Treatment - Earning a Return	5,129	[1]
Receiving Regulatory Treatment - Not Earning a Return	0
Pending Regulatory Treatment	0
Total regulatory assets	5,129		5,504
SPPC [Member] | Ely Energy Center [Member]
Regulatory assets
Remaining Amortization Period	-
Receiving Regulatory Treatment - Earning a Return	0	[1]
Receiving Regulatory Treatment - Not Earning a Return	0
Pending Regulatory Treatment	11,593	[2]
Total regulatory assets	11,593		0
SPPC [Member] | Renewable Energy Programs Regulatory Asset [Member]
Regulatory assets
Remaining Amortization Period	2013
Receiving Regulatory Treatment - Earning a Return	18,898	[1]
Receiving Regulatory Treatment - Not Earning a Return	0
Pending Regulatory Treatment	0
Total regulatory assets	18,898		2,627
SPPC [Member] | Risk Management [Member]
Regulatory assets
Remaining Amortization Period	-
Receiving Regulatory Treatment - Earning a Return	0	[1]
Receiving Regulatory Treatment - Not Earning a Return	0
Pending Regulatory Treatment	0
Total regulatory assets	0		10,465
SPPC [Member] | Other Costs [Member]
Regulatory assets
Remaining Amortization Period	Various thru 2031
Receiving Regulatory Treatment - Earning a Return	10,905	[1]
Receiving Regulatory Treatment - Not Earning a Return	12,080
Pending Regulatory Treatment	2,328	[2],[3]
Total regulatory assets	25,313		38,457
SPPC [Member] | Subtotal - excluding pensions [Member]
Regulatory assets
Receiving Regulatory Treatment - Earning a Return	115,909	[1]
Receiving Regulatory Treatment - Not Earning a Return	192,806
Pending Regulatory Treatment	24,423
SPPC [Member] | Pensions [Member]
Regulatory assets
Receiving Regulatory Treatment - Earning a Return	0	[1]
Receiving Regulatory Treatment - Not Earning a Return	104,159
Pending Regulatory Treatment	$ 0

[1] Earning a return includes either a carrying charge on the asset/liability balance, or a return as a component of rate base.

[2] Pending regulatory treatment includes either amounts which have prior regulatory precedent or have been approved and are subject to prudency review.

[3] Assets which are allowed to earn a carrying charge until included in rates. Reference Note 1, Summary of Significant Accounting Policies, Equity Carrying Charges