Investments (Details-3) - ATC [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity method investment summarized income statement [Abstract]
Operating revenues	$ 650,806	$ 615,836	$ 635,033
Operating expenses	(322,517)	(319,321)	(307,451)
Other income, net	3,225	1,176	117
Interest expense, net	(98,758)	(97,250)	(88,970)
Earnings before members' income taxes	232,756	200,441	$ 238,729
Equity method investment balance sheet assets [Abstract]
Current assets	75,790	80,520
Noncurrent assets	4,312,893	3,948,265
Total assets	4,388,683	4,028,785
Equity method investment balance sheet liabilities [Abstract]
Current liabilities	495,126	330,248
Long-term debt	1,865,302	1,790,718
Other noncurrent liabilities	271,495	244,991
Members' equity	1,756,760	1,662,828
Total members' equity and liabilities	$ 4,388,683	$ 4,028,785