Investment in ATC (Details-2) (ATC [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
ATC [Member]
Equity method investment summarized income statement [Line Items]
Operating revenues	$ 150,303	$ 142,741	$ 450,136	$ 420,562
Operating expenses	(68,813)	(66,413)	(210,139)	(192,491)
Other expense, net	(5)	(173)	(832)	(665)
Interest expense, net	(20,983)	(19,517)	(61,260)	(60,893)
Earnings before members' income taxes	$ 60,502	$ 56,638	$ 177,905	$ 166,513