Investments (Details-3) (ATC [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
ATC [Member]
Equity method investment summarized income statement [Abstract]
Operating revenues	$ 626,336	$ 603,254	$ 567,174
Operating expenses	(295,069)	(280,999)	(261,568)
Other income (expense)	831	(2,533)	(1,332)
Interest expense, net	(84,484)	(82,296)	(80,359)
Earnings before members' income taxes	247,614	237,426	223,915
Equity method investment balance sheet assets [Abstract]
Current assets	80,715	63,134	58,671
Noncurrent assets	3,509,517	3,274,704	3,053,742
Total assets	3,590,232	3,337,838	3,112,413
Equity method investment balance sheet liabilities [Abstract]
Current liabilities	381,467	251,541	298,473
Long-term debt	1,550,000	1,550,000	1,400,005
Other noncurrent liabilities	126,167	95,829	82,647
Members' equity	1,532,598	1,440,468	1,331,288
Total members' equity and liabilities	$ 3,590,232	$ 3,337,838	$ 3,112,413