Investment in ATC and ATC Holdco (Details-2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 186,801	$ 177,715
Operating expenses	(95,211)	(90,387)
Other income, net	375	260
Interest expense, net	(28,888)	(29,118)
Earnings before members' income taxes	63,077	58,470
ATC [Member]
Related Party Transaction [Line Items]
Related party expenses	7,700	$ 7,600
Due from related parties	$ 1,300		$ 1,600