Investment in ATC and ATC Holdco (Details-2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 171,123	$ 158,126	$ 522,402	$ 476,591
Operating expenses	(85,063)	(80,271)	(250,116)	(241,034)
Other income, net	796	1,128	1,615	2,563
Interest expense, net	(28,273)	(24,624)	(81,188)	(73,714)
Earnings before members' income taxes	58,583	54,359	192,713	164,406
ATC [Member]
Related Party Transaction [Line Items]
Related party expenses	7,300	$ 7,400	21,900	$ 22,100
Due from related parties	$ 100		$ 100		$ 100