Regulatory Assets and Liabilities, Regulatory Liabilities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 87,723		$ 83,759
Regulatory Liability, Noncurrent	115,188		81,504
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	0		0
Regulatory Liability, Noncurrent	68,106		53,006
Regulatory liability, remaining amortization period	Plant lives
Deferred Electric Energy Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	53,971		55,395
Regulatory Liability, Noncurrent	0		0
Regulatory liability, remaining amortization period	Less than one year
Contract Valuation Adjustments
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	20,211	[1]	14,243	[1]
Regulatory Liability, Noncurrent	2,521	[1]	6,849	[1]
Regulatory liability, remaining amortization period	Term of related contract
Gain From Asset Sales
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	2,577		11,172
Regulatory Liability, Noncurrent	4,468		4,201
Regulatory liability, remaining amortization period	Various
Conservation Programs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	1,425	[2]	1,465	[2]
Regulatory Liability, Noncurrent	0	[2]	0	[2]
Regulatory liability, remaining amortization period	Less than one year
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	9,539		1,484
Regulatory Liability, Noncurrent	$ 40,093		$ 17,448
Regulatory liability, remaining amortization period	Various

[1] Includes the fair value of certain long-term PPAs used to meet energy capacity requirements.

[2] Includes costs for conservation programs as well as incentives allowed in certain jurisdictions.