Investments (Details-3) - ATC [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity method investment summarized income statement [Abstract]
Operating revenues	$ 690,510	$ 721,672	$ 650,806
Operating expenses	(358,703)	(346,308)	(323,947)
Other income, net	2,405	7,402	5,361
Interest expense, net	(110,725)	(110,138)	(99,464)
Earnings before members' income taxes	223,487	272,628	$ 232,756
Equity method investment balance sheet assets [Abstract]
Current assets	87,250	87,730
Noncurrent assets	4,928,793	4,598,919
Total assets	5,016,043	4,686,649
Equity method investment balance sheet liabilities [Abstract]
Current liabilities	640,040	767,248
Long-term debt	2,013,997	1,790,590
Other noncurrent liabilities	295,281	240,286
Members' equity	2,066,725	1,888,525
Total members' equity and liabilities	$ 5,016,043	$ 4,686,649