Regulatory Matters (Narrative) (Details) - USD ($) $ in Thousands		1 Months Ended
	Mar. 28, 2013	Nov. 30, 2014	Dec. 31, 2015	Dec. 31, 2014
Regulated Asset Liability [Line Items]
Regulatory Liabilities			$ (333,883)	$ (333,628)
Regulatory Assets			579,632	579,000
Refundable gas costs			17,900	3,900
UTC [Member]
Regulated Asset Liability [Line Items]
Decoupling Maximum Rate Increase Request		0.00%
IPUC [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Return on Equity, Percentage	9.80%
Power Deferrals Regulatory Asset [Member]
Regulated Asset Liability [Line Items]
Regulatory Assets			933	8,291
Power Deferrals Regulatory Asset [Member] | IDAHO
Regulated Asset Liability [Line Items]
Regulatory Assets			200	8,300
Power Deferrals [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities			(18,747)	(14,186)
Power Deferrals [Member] | WASHINGTON
Regulated Asset Liability [Line Items]
Regulatory Liabilities			(18,000)	(14,200)
Revenue Subject to Refund [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities			(12,237)	(10,131)
Revenue Subject to Refund [Member] | WASHINGTON
Regulated Asset Liability [Line Items]
Regulatory Liabilities			(3,400)
Revenue Subject to Refund [Member] | IDAHO
Regulated Asset Liability [Line Items]
Regulatory Liabilities			$ (8,800)	$ (10,100)