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		$ 31,541	$ 52,006
Regulatory Asset, Noncurrent		301,814	323,305
Past expenditures not currently earning a return		$ 25,000	53,000
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current	[1]	$ 15,632	17,256
Regulatory Asset, Noncurrent	[1]	$ 223,122	240,980
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		$ 39,368	36,878
Net AROs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Plant lives
Regulatory Asset, Current	[2]	$ 0	0
Regulatory Asset, Noncurrent	[2]	$ 23,014	21,689
Conservation Programs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		One to three years
Regulatory Asset, Current	[3]	$ 5,137	3,451
Regulatory Asset, Noncurrent	[3]	$ 3,859	5,020
Renewable Resources and Environmental Initiatives
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		One to two years
Regulatory Asset, Current		$ 3,740	6,726
Regulatory Asset, Noncurrent		$ 2,019	5,124
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Term of related debt
Regulatory Asset, Current		$ 850	1,104
Regulatory Asset, Noncurrent		$ 1,743	2,594
Deferred Income Tax Adjustment
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Typically plant lives
Regulatory Asset, Current		$ 0	0
Regulatory Asset, Noncurrent		$ 170	302
Recoverable Electric Energy Costs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Less than one year
Regulatory Asset, Current		$ 12	513
Regulatory Asset, Noncurrent		$ 0	0
Texas Surcharge [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Less than one year
Regulatory Asset, Current		$ 0	15,388
Regulatory Asset, Noncurrent		$ 0	0
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current		$ 6,170	7,568
Regulatory Asset, Noncurrent		$ 8,519	$ 10,718

[1] Includes the non-qualified pension plan.

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

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