CONSOLIDATED STATEMENTS OF REVENUES AND EXPENSES - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating revenues:
Total operating revenues	$ 1,430,292	$ 1,480,113	$ 1,434,196
Operating expenses:
Fuel	440,214	502,904	473,184
Production	410,328	417,391	401,374
Depreciation and amortization	243,512	233,284	224,098
Purchased power	68,556	63,468	59,996
Accretion	50,473	38,090	36,674
Total operating expenses	1,213,083	1,255,137	1,195,326
Operating margin	217,209	224,976	238,870
Other income:
Investment income	59,182	60,055	56,122
Amortization of deferred gains	1,788	1,788	1,788
Allowance for equity funds used during construction	771	1,006	784
Other	2,448	5,413	6,291
Total other income	64,189	68,262	64,985
Interest charges:
Interest expense	403,244	381,242	374,345
Allowance for debt funds used during construction	(187,954)	(151,643)	(134,319)
Amortization of debt discount and expense	11,647	12,440	12,552
Net interest charges	226,937	242,039	252,578
Net margin	54,461	51,199	51,277
Members
Operating revenues:
Total operating revenues	1,429,852	1,479,379	1,433,830
Non-Members
Operating revenues:
Total operating revenues	$ 440	$ 734	$ 366