Regulatory Assets and Liabilities, Regulatory Assets (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 16,146	$ 20,036
Regulatory Asset, Noncurrent		289,196	280,693
Past expenditures not currently earning a return		$ 1,000	12,100
Environmental Remediation Costs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current		$ 6,702	4,376
Regulatory Asset, Noncurrent		$ 160,699	147,793
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current	[1]	$ 6,415	6,837
Regulatory Asset, Noncurrent	[1]	$ 86,778	91,601
Recoverable Deferred Taxes on AFUDC Recorded in Plant
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Plant lives
Regulatory Asset, Current		$ 0	0
Regulatory Asset, Noncurrent		$ 20,586	16,711
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Term of related debt
Regulatory Asset, Current		$ 803	801
Regulatory Asset, Noncurrent		$ 4,134	4,936
State Commission Adjustments
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Plant lives
Regulatory Asset, Current		$ 724	488
Regulatory Asset, Noncurrent		$ 12,945	11,650
Deferred Income Tax Adjustment
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Typically plant lives
Regulatory Asset, Current		$ 0	0
Regulatory Asset, Noncurrent		$ 2,250	1,514
Recoverable Purchased Natural Gas And Electric Energy Costs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Less than one year
Regulatory Asset, Current		$ 1,032	6,946
Regulatory Asset, Noncurrent		$ 0	0
Monticello EPU Cost Deferral
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		N/A
Regulatory Asset, Current		$ 0	0
Regulatory Asset, Noncurrent		$ 0	5,237
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current		$ 470	588
Regulatory Asset, Noncurrent		$ 1,804	$ 1,251

[1] Includes the non-qualified pension plan.