Investment in ATC (Details-2) (ATC [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 163,643	$ 160,480	$ 486,970	$ 464,345
Operating expenses	(76,561)	(77,595)	(229,589)	(217,240)
Other income (expense), net	693	917	1,403	585
Interest expense, net	(22,204)	(21,136)	(66,442)	(63,232)
Earnings before members' income taxes	$ 65,571	$ 62,666	$ 192,342	$ 184,458