Fair Value Measurements - NPC - Debt (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value		$ 37,498	$ 35,193
Sierra Pacific Power Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value		1,120	1,120
Nevada Power Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, carrying value		2,601	2,600
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value		41,579	40,522
Level 2 [Member] | Sierra Pacific Power Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value		1,186	1,221
Level 2 [Member] | Nevada Power Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value		2,999	3,088
Recurring [Member] | Money market mutual funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities		413	685
Recurring [Member] | Money market mutual funds [Member] | Sierra Pacific Power Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	[1]	30
Recurring [Member] | Money market mutual funds [Member] | Nevada Power Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	[2]	91
Recurring [Member] | Money market mutual funds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities		413	685
Recurring [Member] | Money market mutual funds [Member] | Level 1 [Member] | Sierra Pacific Power Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	[1]	30
Recurring [Member] | Money market mutual funds [Member] | Level 1 [Member] | Nevada Power Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	[2]	91
Recurring [Member] | Money market mutual funds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities		0	0
Recurring [Member] | Money market mutual funds [Member] | Level 2 [Member] | Sierra Pacific Power Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	[1]	0
Recurring [Member] | Money market mutual funds [Member] | Level 2 [Member] | Nevada Power Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	[2]	0
Recurring [Member] | Money market mutual funds [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities		0	$ 0
Recurring [Member] | Money market mutual funds [Member] | Level 3 [Member] | Sierra Pacific Power Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	[1]	0
Recurring [Member] | Money market mutual funds [Member] | Level 3 [Member] | Nevada Power Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	[2]	$ 0

[1] (1)Amounts are included in cash and cash equivalents on the Consolidated Balance Sheets. The fair value of these money market mutual funds approximates cost.

[2] Amounts are included in cash and cash equivalents on the Consolidated Balance Sheets. The fair value of these money market mutual funds approximates cost.