Schedule III - Electric Plant and Accumulated Depreciation - Components of Electric Plant and Accumulated Depreciation (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Real Estate And Accumulated Depreciation [Line Items]
Gross Amount Carried at Period Close	$ 1,901,960	$ 1,675,295	$ 1,447,247
Accumulated Depreciation	(261,140)	$ (240,764)	$ (220,101)
Stanton Brady And Celeste Assets
Real Estate And Accumulated Depreciation [Line Items]
Encumbrances	391,736
Initial Cost to Company	901,636
Gross Amount Carried at Period Close	901,636
Accumulated Depreciation	(134,121)
McAllen Assets
Real Estate And Accumulated Depreciation [Line Items]
Encumbrances	67,242
Initial Cost to Company	157,124
Gross Amount Carried at Period Close	157,124
Accumulated Depreciation	(25,379)
Competitive Renewable Energy Zones Assets
Real Estate And Accumulated Depreciation [Line Items]
Encumbrances	329,680
Initial Cost to Company	685,954
Gross Amount Carried at Period Close	685,954
Accumulated Depreciation	(40,023)
Stanton Transmission Loop Assets
Real Estate And Accumulated Depreciation [Line Items]
Encumbrances	16,578
Initial Cost to Company	92,318
Gross Amount Carried at Period Close	92,318
Accumulated Depreciation	(59,837)
ERCOT Transmission Assets
Real Estate And Accumulated Depreciation [Line Items]
Encumbrances	32,230
Initial Cost to Company	64,928
Gross Amount Carried at Period Close	64,928
Accumulated Depreciation	$ (1,780)