Investment in ATC (Details-2) (ATC [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
ATC [Member]
Equity method investment summarized income statement [Line Items]
Operating revenues	$ 152,171	$ 138,203	$ 299,833	$ 277,820
Operating expenses	(71,760)	(62,951)	(141,326)	(126,077)
Other expense, net	(327)	(134)	(827)	(492)
Interest expense, net	(20,776)	(19,479)	(40,277)	(41,376)
Earnings before members' income taxes	$ 59,308	$ 55,639	$ 117,403	$ 109,875