Revenue Recognition - Schedule of Sales to Members and Sales to Non-Members (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Revenue Recognition
Total operating revenues	$ 631,461	$ 540,670	$ 1,942,785	$ 1,641,074
Sales to members
Revenue Recognition
Total operating revenues	594,248	515,497	1,864,239	1,613,134
Sales to members | Capacity revenues
Revenue Recognition
Total operating revenues	373,386	362,852	1,209,682	1,120,280
Sales to members | Energy revenues
Revenue Recognition
Total operating revenues	220,862	152,645	654,557	492,854
Sales to non-members
Revenue Recognition
Total operating revenues	37,213	25,173	78,546	27,940
Sales to non-members | Capacity revenues
Revenue Recognition
Total operating revenues	0	0	0	1,572
Sales to non-members | Energy revenues
Revenue Recognition
Total operating revenues	$ 37,213	$ 25,173	$ 78,546	$ 26,368