Revenue Recognition - Sales to Members & Non-Members (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Revenue Recognition
Total operating revenues	$ 536,314	$ 389,453
Sales to members
Revenue Recognition
Total operating revenues	535,370	387,653
Sales to members | Capacity revenues
Revenue Recognition
Total operating revenues	355,718	242,043
Sales to members | Energy revenues
Revenue Recognition
Total operating revenues	179,652	145,610
Sales to non-members
Revenue Recognition
Total operating revenues	944	1,800
Sales to non-members | Capacity revenues
Revenue Recognition
Total operating revenues	786	763
Sales to non-members | Energy revenues
Revenue Recognition
Total operating revenues	$ 158	$ 1,037