Investment in ATC and ATC Holdco (Details-2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 170,341	$ 171,123	$ 501,276	$ 522,402
Operating expenses	(87,959)	(85,372)	(264,326)	(251,042)
Other income, net	439	1,272	2,070	3,043
Interest expense, net	(27,754)	(28,440)	(82,411)	(81,690)
Earnings before members' income taxes	55,067	58,583	156,609	192,713
ATC [Member]
Related Party Transaction [Line Items]
Related party expenses	7,200	$ 7,300	21,700	$ 21,900
Due from related parties	$ 100		$ 100		$ 200