Investments (Details-3) - ATC [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity method investment summarized income statement [Abstract]
Operating revenues	$ 721,672	$ 650,806	$ 615,836
Operating expenses	(344,935)	(322,517)	(319,321)
Other income, net	5,285	3,225	1,176
Interest expense, net	(109,394)	(98,758)	(97,250)
Earnings before members' income taxes	272,628	232,756	$ 200,441
Equity method investment balance sheet assets [Abstract]
Current assets	87,730	75,790
Noncurrent assets	4,598,919	4,312,893
Total assets	4,686,649	4,388,683
Equity method investment balance sheet liabilities [Abstract]
Current liabilities	767,248	495,126
Long-term debt	1,790,590	1,865,302
Other noncurrent liabilities	240,286	271,495
Members' equity	1,888,525	1,756,760
Total members' equity and liabilities	$ 4,686,649	$ 4,388,683