Investment in ATC (Details-2) (ATC [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 160,480	$ 150,303	$ 464,345	$ 450,136
Operating expenses	(77,595)	(68,813)	(217,240)	(210,139)
Other income (expense), net	917	(5)	585	(832)
Interest expense, net	(21,136)	(20,983)	(63,232)	(61,260)
Earnings before members' income taxes	$ 62,666	$ 60,502	$ 184,458	$ 177,905