Regulatory Matters (Narrative) (Details) (USD $)	0 Months Ended	12 Months Ended			1 Months Ended		0 Months Ended			1 Months Ended	0 Months Ended	1 Months Ended
	May 10, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 30, 2014	Dec. 31, 2012	Dec. 26, 2012	Feb. 04, 2014	Mar. 28, 2013	Sep. 30, 2014	Jan. 21, 2014	Feb. 25, 2015	Jan. 31, 2015	Mar. 31, 2014
Regulated Asset Liability [Line Items]
Regulatory Liabilities		$ (333,628,000)	$ (354,281,000)
Wind development project costs	4,300,000
Deadband amount		4,000,000
Natural gas cost deferrals and recovery percentage		100.00%
Natural gas cost deferrals and recovery percentage recovered		90.00%
Refundable gas costs		3,900,000	12,100,000
Payments to Acquire Property, Plant, and Equipment		325,516,000	294,363,000	271,187,000
Payments to Acquire Other Property, Plant, and Equipment		325,922,000	294,734,000	271,853,000
Estimated Capital Expenditures Next Twelve Months		375,000,000
Estimated Capital Expenditures Year Two		350,000,000
Ownership interest		15.00%
Electric Transmission Revenue Credit	4,200,000
Maximum [Member]
Regulated Asset Liability [Line Items]
Annual power supply cost variances		10,000,000
Minimum [Member]
Regulated Asset Liability [Line Items]
Annual power supply cost variances		4,000,000
Four Million Deadband [Member]
Regulated Asset Liability [Line Items]
Expense or benefit to the company		100.00%
Higher by $4 million to $10 million [Member]
Regulated Asset Liability [Line Items]
Deferred for future surcharge or rebate to customers		50.00%
Expense or benefit to the company		50.00%
Lower by $4 million to $10 million [Member]
Regulated Asset Liability [Line Items]
Deferred for future surcharge or rebate to customers		75.00%
Expense or benefit to the company		25.00%
Higher or lower by over $10 million [Member]
Regulated Asset Liability [Line Items]
Annual power supply cost variances		10,000,000
Deferred for future surcharge or rebate to customers		90.00%
Expense or benefit to the company		10.00%
UTC [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Return on Equity, Percentage						9.80%
Public Utilities, Approved Equity Capital Structure, Percentage						47.00%
Public Utilities, Approved Rate Increase (Decrease), Percentage						7.64%
Decoupling Maximum Rate Increase Request					0.00%
Public Utilities, Approved Rate of Return					0.00%
Expense Or Benefit to the Company Rate of Return Deferral, Percentage					0.00%
UTC [Member] | 2.0 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage							2.00%
UTC [Member] | 2.5 Percent [Member]
Regulated Asset Liability [Line Items]
Expiring Rebates, Percentage		2.80%
Renewable Energy Credit Rebate To Customers		8,600,000
Public Utilities, Approved Rate Increase (Decrease), Percentage					2.50%
Power Supply Costs Update					5,300,000
Public Utilities, Approved Rate Increase (Decrease), Amount					12,300,000
Renewable Energy Credit Rebate To Customers, Percentage		1.20%
UTC [Member] | Five Point Six Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage					5.60%
Public Utilities, Approved Rate Increase (Decrease), Amount					8,500,000
UTC [Member] | 3.8 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage								3.80%
Public Utilities, Requested Rate Increase (Decrease), Amount								18,200,000
UTC [Member] | 8.1 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage								8.10%
Public Utilities, Requested Rate Increase (Decrease), Amount								12,100,000
UTC [Member] | 3.0 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage						3.00%
UTC [Member] | 0.9 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage						0.90%
Public Utilities, Approved Rate Increase (Decrease), Amount						1,400,000
UTC [Member] | 3.6 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage						3.60%
Public Utilities, Approved Rate Increase (Decrease), Amount						5,300,000
UTC [Member] | 1.6 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease) Related to Rebates, Percentage					1.60%
UTC [Member] | 2015 | 2.5 Percent [Member]
Regulated Asset Liability [Line Items]
Power cost, deferral balance					8,300,000
UTC [Member] | 2013 [Member] | 2.0 Percent [Member]
Regulated Asset Liability [Line Items]
Power cost, deferral balance							4,400,000
UTC [Member] | 2014 [Member] | 2.0 Percent [Member]
Regulated Asset Liability [Line Items]
Power cost, deferral balance							9,000,000
UTC [Member] | 2014 [Member] | 3.0 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount						14,000,000
UTC [Member] | 2013 [Member] | 3.0 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount						13,600,000
UTC [Member] | Software and Software Development Costs [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Requested Capital Costs, Amount					2,000,000
IPUC [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Return on Equity, Percentage									9.80%
Estimated Increase in Pre-Tax Income										3,700,000
Public Utilities, Approved Equity Capital Structure, Percentage									50.00%
IPUC [Member] | Maximum [Member]
Regulated Asset Liability [Line Items]
Return on Equity Deadband										0.098
IPUC [Member] | Minimum [Member]
Regulated Asset Liability [Line Items]
Return on Equity Deadband										0.095
IPUC [Member] | 1.9 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage									1.90%
IPUC [Member] | 2013 And 2014 [Member] | 2.0 Percent [Member]
Regulated Asset Liability [Line Items]
Power cost, deferral balance									1,600,000
IPUC [Member] | 2013 And 2014 [Member] | 3.1 Percent [Member]
Regulated Asset Liability [Line Items]
Power cost, deferral balance									3,900,000
IPUC [Member] | April Twenty Thirteen [Member] | 4.9 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage									4.90%
Public Utilities, Approved Rate Increase (Decrease), Amount									3,100,000
IPUC [Member] | October Twenty Thirteen [Member] | 2.0 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage									2.00%
Public Utilities, Approved Rate Increase (Decrease), Amount									1,300,000
IPUC [Member] | October Twenty Thirteen [Member] | 0.3 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage									0.30%
IPUC [Member] | October Twenty Thirteen [Member] | 3.1 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage									3.10%
Public Utilities, Approved Rate Increase (Decrease), Amount									7,800,000
O.P.U.C [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Return on Equity, Percentage											9.65%
Public Utilities, Approved Equity Capital Structure, Percentage											48.00%
Public Utilities, Approved Rate Increase (Decrease), Percentage											7.47%
Public Utilities, Requested Rate of Return on Rate Base										7.77%
Public Utilities, Requested Return on Equity, Percentage										9.90%
Public Utilities, Requested Equity Capital Structure, Percentage										51.00%
O.P.U.C [Member] | Nine Point Three Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage										9.30%
Public Utilities, Requested Rate Increase (Decrease), Amount										9,100,000
O.P.U.C [Member] | February Twenty Fourteen [Member] | 4.4 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage											4.40%
Public Utilities, Approved Rate Increase (Decrease), Amount											3,800,000
O.P.U.C [Member] | November Twenty Fourteen [Member] | 1.55 Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage											1.60%
Public Utilities, Approved Rate Increase (Decrease), Amount					300,000						1,400,000
Bonneville Power Administration [Member]
Regulated Asset Liability [Line Items]
Past Use of Transmission System Settlement	11,700,000
WASHINGTON
Regulated Asset Liability [Line Items]
Total net deferred power costs		14,200,000	17,900,000
Wind development project costs	2,500,000
WASHINGTON | Bonneville Power Administration [Member]
Regulated Asset Liability [Line Items]
Past Use of Transmission System Settlement	7,600,000
Past Use of Transmission System Settlement Less Wind Project Costs	5,100,000
IDAHO
Regulated Asset Liability [Line Items]
Total net deferred power costs		8,300,000	5,100,000
Wind development project costs									1,700,000
Deferred for future surcharge or rebate to customers		90.00%
IDAHO | Bonneville Power Administration [Member]
Regulated Asset Liability [Line Items]
Past Use of Transmission System Settlement									4,100,000
OREGON
Regulated Asset Liability [Line Items]
Natural gas cost deferrals and recovery percentage		100.00%
Annual [Member] | Bonneville Power Administration [Member]
Regulated Asset Liability [Line Items]
Electrical Transmission Revenue	3,200,000
Annual [Member] | WASHINGTON | Bonneville Power Administration [Member]
Regulated Asset Liability [Line Items]
Electrical Transmission Revenue	2,100,000
Deferred Maintenance Costs [Member]
Regulated Asset Liability [Line Items]
Period of years with deferred costs amortized		2017
Percent [Member]
Regulated Asset Liability [Line Items]
Deferred for future surcharge or rebate to customers		50.00%
Renewable Energy Credit Rebates [Member] | UTC [Member] | 2.5 Percent [Member]
Regulated Asset Liability [Line Items]
Period of years with deferred costs amortized		18
Revenue Subject to Refund [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities		(7,724,000)
Subsequent Event [Member] | UTC [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Requested Rate of Return on Rate Base												7.46%
Public Utilities, Requested Return on Equity, Percentage												9.90%
Public Utilities, Requested Equity Capital Structure, Percentage												48.00%
Subsequent Event [Member] | UTC [Member] | Six Point Six Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage												6.60%
Public Utilities, Requested Rate Increase (Decrease), Amount												33,200,000
Subsequent Event [Member] | UTC [Member] | Seven Point Zero Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage												7.00%
Public Utilities, Requested Rate Increase (Decrease), Amount												12,000,000
Subsequent Event [Member] | O.P.U.C [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Requested Rate of Return on Rate Base													7.52%
Public Utilities, Requested Return on Equity, Percentage													9.50%
Public Utilities, Requested Equity Capital Structure, Percentage													51.00%
Subsequent Event [Member] | O.P.U.C [Member] | Six Point One Percent [Member]
Regulated Asset Liability [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage													6.10%
Public Utilities, Requested Rate Increase (Decrease), Amount													6,100,000
Public Utilities, Requested Rate Increase, Amount after Offsets													5,000,000
Public Utilities, Approved Rate Increase (Decrease), Amount													300,000
Public Utilities Approved Credit to Customers													800,000
Electric [Member] | IPUC [Member]
Regulated Asset Liability [Line Items]
Expiring Rebates, Percentage		1.30%
Electric [Member] | Revenue Subject to Refund [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities		(7,500,000)	(3,914,000)											(1,900,000)
Revenues Subject to Refund			(2,000,000)
Natural Gas [Member] | IPUC [Member]
Regulated Asset Liability [Line Items]
Expiring Rebates, Percentage		1.70%
Natural Gas [Member] | Revenue Subject to Refund [Member]
Regulated Asset Liability [Line Items]
Regulatory Liabilities		$ (200,000)	$ (400,000)