Investment in ATC and ATC Holdco (Details-2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 203,071	$ 182,212	$ 389,872	$ 359,927
Operating expenses	(97,511)	(93,573)	(192,722)	(183,960)
Other income, net	906	287	1,281	547
Interest expense, net	(26,258)	(28,838)	(55,146)	(57,956)
Earnings before members' income taxes	80,208	60,088	143,285	118,558
ATC [Member]
Related Party Transaction [Line Items]
Related party expenses	7,600	$ 7,600	15,300	$ 15,200
Due from related parties	$ 1,500		$ 1,500		$ 1,600