Regulatory Assets and Liabilities, Regulatory Assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Regulatory Assets [Line Items]
Regulatory Asset, Current	$ 150,163,000		$ 143,689,000
Regulatory Asset, Noncurrent	826,037,000		934,728,000
Past expenditures not currently earning a return	136,000,000		135,000,000
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory Asset, Current	45,504,000	[1]	38,219,000	[1]
Regulatory Asset, Noncurrent	447,752,000	[1]	613,391,000	[1]
Regulatory asset, remaining amortization period	Various
Non Qualified Pension Plan
Regulatory Assets [Line Items]
Regulatory Asset, Current	400,000		400,000
Regulatory Asset	4,100,000		4,400,000
Recoverable Deferred Taxes on AFUDC Recorded in Plant
Regulatory Assets [Line Items]
Regulatory Asset, Current	0		0
Regulatory Asset, Noncurrent	116,477,000		100,060,000
Regulatory asset, remaining amortization period	Plant lives
Contract Valuation Adjustments
Regulatory Assets [Line Items]
Regulatory Asset, Current	3,620,000	[2]	3,763,000	[2]
Regulatory Asset, Noncurrent	16,474,000	[2]	20,094,000	[2]
Regulatory asset, remaining amortization period	Term of related contract
Depreciation Differences
Regulatory Assets [Line Items]
Regulatory Asset, Current	10,917,000		5,274,000
Regulatory Asset, Noncurrent	95,844,000		50,057,000
Regulatory asset, remaining amortization period, minimum	1 year
Regulatory asset, remaining amortization period, maximum	17 years
Net AROs
Regulatory Assets [Line Items]
Regulatory Asset, Current	0	[3]	0	[3]
Regulatory Asset, Noncurrent	39,986,000	[3]	40,184,000	[3]
Regulatory asset, remaining amortization period	Plant lives
Conservation Programs
Regulatory Assets [Line Items]
Regulatory Asset, Current	13,883,000	[4]	16,887,000	[4]
Regulatory Asset, Noncurrent	19,430,000	[4]	28,528,000	[4]
Regulatory asset, remaining amortization period, minimum	1 year
Regulatory asset, remaining amortization period, maximum	5 years
Gas Pipeline Inspection Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current	5,416,000		5,416,000
Regulatory Asset, Noncurrent	14,978,000		15,220,000
Regulatory asset, remaining amortization period, minimum	1 year
Regulatory asset, remaining amortization period, maximum	4 years
Renewable Resources and Environmental Initiatives
Regulatory Assets [Line Items]
Regulatory Asset, Current	22,325,000		42,647,000
Regulatory Asset, Noncurrent	0		1,807,000
Regulatory asset, remaining amortization period, minimum	1 year
Regulatory asset, remaining amortization period, maximum	2 years
Purchased Power Agreements
Regulatory Assets [Line Items]
Regulatory Asset, Current	0		0
Regulatory Asset, Noncurrent	30,069,000		28,164,000
Regulatory asset, remaining amortization period	Term of related contract
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory Asset, Current	1,572,000		1,964,000
Regulatory Asset, Noncurrent	9,804,000		11,376,000
Regulatory asset, remaining amortization period	Term of related debt
Recoverable Purchased Natural Gas and Electric Energy Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current	18,022,000		15,007,000
Regulatory Asset, Noncurrent	0		0
Regulatory asset, remaining amortization period	Less than one year
Property Tax
Regulatory Assets [Line Items]
Regulatory Asset, Current	18,427,000		6,005,000
Regulatory Asset, Noncurrent	30,626,000		12,010,000
Regulatory asset, remaining amortization period, minimum	1 year
Regulatory asset, remaining amortization period, maximum	3 years
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory Asset, Current	10,477,000		8,507,000
Regulatory Asset, Noncurrent	$ 4,597,000		$ 13,837,000
Regulatory asset, remaining amortization period	Various

[1] Includes $4.1 million and $4.4 million of regulatory assets related to the nonqualified pension plan, of which $0.4 million is included in the current asset at Dec. 31, 2013 and 2012, respectively.

[2] Includes the fair value of certain long-term PPAs used to meet energy capacity requirements and valuation adjustments on natural gas commodity purchases.

[3] Includes amounts recorded for future recovery of AROs.

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