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SEGMENT INFORMATION (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2010
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Segment Reporting Information [Line Items]                      
Operating Revenues $ 609,597 $ 1,017,796 $ 674,931 $ 640,983 $ 655,011 $ 1,128,039 $ 782,683 $ 714,489 $ 2,943,307 $ 3,280,222 $ 3,585,798
Energy Costs:                      
Fuel for power generation                 680,585 821,484 881,768
Purchased power                 633,874 648,881 758,736
Gas purchased for resale                 125,155 137,702 153,607
Deferred energy                 (83,333) 113,107 289,076
Energy efficiency program costs                 43,537 0 0
Total Costs                 1,399,818 1,721,174 2,083,187
Gross Margin                 1,543,489 1,559,048 1,502,611
Other Cost and Expense, Operating                 411,115 414,241 453,413
Utilities Operating Expense, Maintenance                 103,307 104,567 102,309
Depreciation, Depletion and Amortization, Nonproduction                 357,937 333,059 321,921
Taxes, Other                 60,465 62,746 60,885
Operating Income 76,684 353,196 106,919 73,866 103,435 343,364 124,730 72,906 610,665 644,435 564,083
Assets 11,635,128       11,669,668       11,635,128 11,669,668  
Capital expenditures                 620,516 629,496 843,132
Unallocated Amount to Segment [Member]
                      
Segment Reporting Information [Line Items]                      
Operating Revenues                 15 23 28
Energy Costs:                      
Fuel for power generation                 0 0 0
Purchased power                 0 0 0
Gas purchased for resale                 0 0 0
Deferred energy                 0 0 0
Energy efficiency program costs                 0    
Total Costs                 0 0 0
Gross Margin                 15 23 28
Other Cost and Expense, Operating                 4,289 3,760 2,699
Utilities Operating Expense, Maintenance                 0 0 0
Depreciation, Depletion and Amortization, Nonproduction                 0 0 0
Taxes, Other                 290 235 197
Operating Income                 (4,564) (3,972) (2,868)
Assets 8,523       20,822       8,523 20,822 (25,053)
Capital expenditures                 0 (13,094) [1] 0
NPC Electric [Member]
                      
Segment Reporting Information [Line Items]                      
Operating Revenues                 2,054,393 2,252,377 2,423,377
Energy Costs:                      
Fuel for power generation                 498,487 588,419 587,647
Purchased power                 477,226 505,239 627,759
Deferred energy                 (16,300) 94,843 207,611
Energy efficiency program costs                 37,292    
Total Costs                 996,705 1,188,501 1,423,017
Gross Margin                 1,057,688 1,063,876 1,000,360
Other Cost and Expense, Operating                 260,127 260,535 279,865
Utilities Operating Expense, Maintenance                 64,320 71,759 71,019
Depreciation, Depletion and Amortization, Nonproduction                 252,191 226,252 215,873
Taxes, Other                 37,254 37,918 37,241
Operating Income                 443,796 467,412 396,362
Assets 8,442,597       8,301,824       8,442,597 8,301,824 8,096,371
Capital expenditures                 475,118 499,374 656,074
SPPC Total [Member]
                      
Segment Reporting Information [Line Items]                      
Operating Revenues                 888,899 1,027,822 1,162,393
Energy Costs:                      
Fuel for power generation                 182,098 233,065 294,121
Purchased power                 156,648 143,642 130,977
Gas purchased for resale                 125,155 137,702 153,607
Deferred energy                 (67,033) 18,264 81,465
Energy efficiency program costs                 6,245    
Total Costs                 403,113 532,673 660,170
Gross Margin                 485,786 495,149 502,223
Other Cost and Expense, Operating                 146,699 149,946 170,849
Utilities Operating Expense, Maintenance                 38,987 32,808 31,290
Depreciation, Depletion and Amortization, Nonproduction                 105,746 106,807 106,048
Taxes, Other                 22,921 24,593 23,447
Operating Income                 171,433 180,995 170,589
Assets 3,184,008       3,347,022       3,184,008 3,347,022 3,342,145
Capital expenditures                 145,398 143,216 187,058
SPPC Electric [Member]
                      
Segment Reporting Information [Line Items]                      
Operating Revenues                 716,417 836,879 957,130
Energy Costs:                      
Fuel for power generation                 182,098 233,065 294,121
Purchased power                 156,648 143,642 130,977
Deferred energy                 (65,445) 8,475 73,829
Energy efficiency program costs                 6,245    
Total Costs                 279,546 385,182 498,927
Gross Margin                 436,871 451,697 458,203
Assets 2,818,927       3,022,257       2,818,927 3,022,257 2,997,116
Capital expenditures                 132,083 131,579 171,036
SPPC Gas [Member]
                      
Segment Reporting Information [Line Items]                      
Operating Revenues                 172,482 190,943 205,263
Energy Costs:                      
Gas purchased for resale                 125,155 137,702 153,607
Deferred energy                 (1,588) 9,789 7,636
Total Costs                 123,567 147,491 161,243
Gross Margin                 48,915 43,452 44,020
Assets 302,062       291,122       302,062 291,122 305,434
Capital expenditures                 13,315 11,637 16,022
Intersegment Elimination [Member]
                      
Energy Costs:                      
Assets $ 63,019       $ 33,643       $ 63,019 $ 33,643 $ 39,595
[1] The capital expenditures for NVE Other at December 31, 2010 includes $13.1 million proceeds from the sale of assets between SPPC and SierraPacific Communications.