Basis of Presentation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenues	$ 1,385,160	$ 1,587,289	$ 1,362,928	$ 1,197,373	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 5,532,750	$ 5,336,776	$ 4,531,552
Receivables, net	873,986				836,605				873,986	836,605
Operating expenses	1,239,270	1,383,578	1,224,674	1,140,303	1,247,992	1,374,329	1,206,262	1,026,973	4,987,825	4,855,556	4,129,829
Net income	112,294	$ 153,078	$ 99,703	25,130	95,065	$ 137,637	$ 61,825	$ 40,926	390,205	335,453	$ 272,318
Accumulated other comprehensive loss	(48,078)				(42,102)				(48,078)	(42,102)
Regulatory liabilities	428,075				$ 447,370				428,075	$ 447,370
Revision of Prior Period, Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenues				(7,700)
Receivables, net				(7,700)
Operating expenses				1,200
Accounts Payable				1,200
Net income	4,400			$ (6,700)
Other Income	6,500
Accumulated other comprehensive loss	(2,700)								(2,700)
Regulatory liabilities	$ (3,800)								$ (3,800)