Long - Term Debt Present Value of Future Minimum Lease Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Year One	$ 1,164,000
Year Two	1,141,878
Year Three	2,452,194
Year Four	1,340,000
Year Five	1,378,000
Long-term Debt, Fair Value	24,813,818	$ 19,059,950
Interest Expense, Debt	837,981	807,382	$ 768,322
Total Long-Term Debt	22,369,776	17,873,655
System Energy [Member]
Year One	100,000
Year Two	134,000
Year Three	250,000
Year Four	0
Year Five	200,000
Long-term Debt, Fair Value	840,540	565,209
Interest Expense, Debt	34,467	35,328	$ 38,424
Total Long-Term Debt	805,274	$ 548,107
Grand Gulf [Member] | System Energy [Member]
Year One	17,188
Year Two	17,188
Year Three	17,188
Year Four	17,188
Year Five	17,188
Long-term Debt, Maturities, Repayments of Principal after Year Five	189,063
Long-term Debt, Fair Value	275,003
Interest Expense, Debt	240,667
Total Long-Term Debt	$ 34,336