Regulatory Assets and Liabilities, Regulatory Assets (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 18,162	$ 16,146
Regulatory Asset, Noncurrent		286,188	289,196
Past expenditures not currently earning a return		$ 0	1,000
Environmental Remediation Costs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current		$ 10,669	6,702
Regulatory Asset, Noncurrent		$ 148,880	160,699
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current	[1]	$ 5,989	6,415
Regulatory Asset, Noncurrent	[1]	$ 93,160	86,778
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		$ 22,345	20,586
State Commission Adjustments
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Plant lives
Regulatory Asset, Current		$ 703	724
Regulatory Asset, Noncurrent		$ 14,008	12,945
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Term of related debt
Regulatory Asset, Current		$ 801	803
Regulatory Asset, Noncurrent		$ 3,333	4,134
Deferred Income Tax Adjustment
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Typically plant lives
Regulatory Asset, Current		$ 0	0
Regulatory Asset, Noncurrent		$ 2,078	2,250
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current		$ 0	1,502
Regulatory Asset, Noncurrent		$ 2,384	$ 1,804

[1] (a) Includes the non-qualified pension plan.