Business Combination - Unaudited Pro Forma Information (Details) - South Central Generating - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Business Acquisition [Line Items]
Operating revenue, net	$ 487,971	$ 469,419	$ 1,250,787	$ 1,263,182
Net income	$ 56,004	$ 71,076	$ 122,771	$ 140,255