Investment in ATC (Details-3) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 158,126	$ 164,515	$ 476,591	$ 482,043
Operating expenses	(80,271)	(78,059)	(241,034)	(238,336)
Other income, net	1,128	585	2,563	566
Interest expense, net	(24,624)	(23,655)	(73,714)	(72,310)
Earnings before members' income taxes	54,359	63,386	164,406	171,963
ATC [Member]
Related Party Transaction [Line Items]
Related party expenses	7,400	$ 7,000	22,100	$ 21,100
Due from related parties	$ 300		$ 300		$ 200