Regulatory Matters Regulatory Matters (Decoupling and Earnings Sharing) (Details) - USD ($) $ in Thousands	Mar. 28, 2013	Dec. 31, 2015	Dec. 31, 2014
Schedule of Income Tax Related Regulatory Assets and Liabilities [Line Items]
Decoupling		$ 579,632	$ 579,000
Regulatory Liabilities		333,883	333,628
Decoupling [Member]
Schedule of Income Tax Related Regulatory Assets and Liabilities [Line Items]
Decoupling		13,312	0
Decoupling [Member] | WASHINGTON
Schedule of Income Tax Related Regulatory Assets and Liabilities [Line Items]
Decoupling		10,900
Revenue Subject to Refund [Member]
Schedule of Income Tax Related Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities		12,237	10,131
Revenue Subject to Refund [Member] | WASHINGTON
Schedule of Income Tax Related Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities		3,400
Revenue Subject to Refund [Member] | IDAHO
Schedule of Income Tax Related Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities		$ 8,800	$ 10,100
IPUC [Member]
Schedule of Income Tax Related Regulatory Assets and Liabilities [Line Items]
Public Utilities, Approved Return on Equity, Percentage	9.80%