Electric Plant and Accumulated Depreciation - Components of Electric Plant and Accumulated Depreciation (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Real Estate And Accumulated Depreciation [Line Items]
Gross Amount Carried at Period Close	$ 1,675,295	$ 1,447,247	$ 1,298,946
Accumulated Depreciation	(240,764)	$ (220,101)	$ (194,569)
Stanton Brady And Celeste Assets
Real Estate And Accumulated Depreciation [Line Items]
Encumbrances	268,620
Initial Cost to Company	713,502
Gross Amount Carried at Period Close	713,502
Accumulated Depreciation	(129,770)
McAllen Assets
Real Estate And Accumulated Depreciation [Line Items]
Encumbrances	61,676
Initial Cost to Company	155,970
Gross Amount Carried at Period Close	155,970
Accumulated Depreciation	(21,943)
Competitive Renewable Energy Zones
Real Estate And Accumulated Depreciation [Line Items]
Encumbrances	294,159
Initial Cost to Company	669,189
Gross Amount Carried at Period Close	669,189
Accumulated Depreciation	(29,960)
Stanton Transmission Loop Assets
Real Estate And Accumulated Depreciation [Line Items]
Encumbrances	15,767
Initial Cost to Company	92,572
Gross Amount Carried at Period Close	92,572
Accumulated Depreciation	(58,309)
E R C O T Transmission Assets
Real Estate And Accumulated Depreciation [Line Items]
Encumbrances	19,917
Initial Cost to Company	44,062
Gross Amount Carried at Period Close	44,062
Accumulated Depreciation	$ (782)