Investments (Details-3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity method investment summarized assets [Abstract]
Total Current Assets	$ 209,519	$ 181,641
Total Assets	2,253,651	2,081,664	$ 1,988,618
Equity method investment summarized liabilities and equity [Abstract]
Total Current Liabilities	190,926	128,820
Total Other Credits	567,422	573,427
Total Liabilities and Capitalization	2,253,651	2,081,664
ATC [Member]
Equity method investment summarized income statement [Abstract]
Operating revenues	758,117	744,371	690,510
Operating expenses	(372,463)	(373,527)	(358,703)
Other income, net	1,922	48	2,405
Interest expense, net	(112,818)	(110,490)	(110,725)
Earnings before members' income taxes	274,758	260,402	$ 223,487
Equity method investment summarized assets [Abstract]
Total Current Assets	92,735	84,635
Total Noncurrent Assets	5,400,538	5,244,220
Total Assets	5,493,273	5,328,855
Equity method investment summarized liabilities and equity [Abstract]
Total Current Liabilities	310,749	502,601
Long-term debt	2,512,246	2,312,799
Total Other Credits	378,205	298,828
Total Equity	2,292,073	2,214,627
Total Liabilities and Capitalization	$ 5,493,273	$ 5,328,855