Investments (Details-3) (ATC [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
ATC [Member]
Equity method investment summarized income statement [Abstract]
Operating revenues	$ 635,033	$ 626,336	$ 603,254
Operating expenses	(307,451)	(295,069)	(280,999)
Other income (expense)	117	831	(2,533)
Interest expense, net	(88,970)	(84,484)	(82,296)
Earnings before members' income taxes	238,729	247,614	237,426
Equity method investment balance sheet assets [Abstract]
Current assets	66,410	80,715	63,134
Noncurrent assets	3,728,675	3,509,517	3,274,704
Total assets	3,795,085	3,590,232	3,337,838
Equity method investment balance sheet liabilities [Abstract]
Current liabilities	313,065	381,467	251,541
Long-term debt	1,701,000	1,550,000	1,550,000
Other noncurrent liabilities	163,818	126,167	95,829
Members' equity	1,617,202	1,532,598	1,440,468
Total members' equity and liabilities	$ 3,795,085	$ 3,590,232	$ 3,337,838