Investments (Details-3) (ATC [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
ATC [Member]
Equity method investment summarized income statement [Abstract]
Operating revenues	$ 567,174	$ 556,741	$ 521,525
Operating expenses	(261,568)	(251,120)	(230,316)
Other expense, net	(1,332)	(885)	(621)
Interest expense, net	(80,359)	(85,067)	(77,223)
Earnings before members' income taxes	223,915	219,669	213,365
Equity method investment balance sheet assets [Abstract]
Current assets	58,671	59,856	51,121
Noncurrent assets	3,053,742	2,888,448	2,767,249
Total assets	3,112,413	2,948,304	2,818,370
Equity method investment balance sheet liabilities [Abstract]
Current liabilities	298,473	428,387	285,494
Long-term debt	1,400,005	1,175,010	1,259,643
Other noncurrent liabilities	82,647	84,940	76,837
Members' equity	1,331,288	1,259,967	1,196,396
Total members' equity and liabilities	$ 3,112,413	$ 2,948,304	$ 2,818,370