Investments (Details-3) - ATC [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Equity method investment summarized income statement [Abstract]
Operating revenues	$ 615,836	$ 635,033	$ 626,336
Operating expenses	(319,321)	(307,451)	(295,069)
Other income, net	1,176	117	831
Interest expense, net	(97,250)	(88,970)	(84,484)
Earnings before members' income taxes	200,441	238,729	247,614
Equity method investment balance sheet assets [Abstract]
Current assets	80,520	66,410	80,715
Noncurrent assets	3,957,576	3,728,675	3,509,517
Total assets	4,038,096	3,795,085	3,590,232
Equity method investment balance sheet liabilities [Abstract]
Current liabilities	330,248	313,065	381,467
Long-term debt	1,800,029	1,701,000	1,550,000
Other noncurrent liabilities	244,991	163,818	126,167
Members' equity	1,662,828	1,617,202	1,532,598
Total members' equity and liabilities	$ 4,038,096	$ 3,795,085	$ 3,590,232