Investment in ATC (Details-2) - ATC [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 164,515	$ 163,643	$ 482,043	$ 486,970
Operating expenses	(78,059)	(76,561)	(238,336)	(229,589)
Other income (expense), net	585	693	566	1,403
Interest expense, net	(23,655)	(22,204)	(72,310)	(66,442)
Earnings before members' income taxes	$ 63,386	$ 65,571	$ 171,963	$ 192,342