Equity - Narrative (Details) $ in Thousands		1 Months Ended		12 Months Ended			13 Months Ended
	May 31, 2025 USD ($)	Jul. 31, 2026 USD ($)	Sep. 30, 2025 USD ($)	May 31, 2026 USD ($) reserve	May 31, 2025 USD ($)	May 31, 2024 USD ($)	May 31, 2026 USD ($) reserve	May 31, 2023 USD ($)
Stockholder's Equity [Line Items]
Stockholders' equity, period increase				$ 209,000
Total equity	$ 3,103,466			3,312,136	$ 3,103,466	$ 3,012,169	$ 3,312,136	$ 2,589,249
Net income (loss)				262,736	140,014	554,316
Patronage capital retirement				$ 52,978	46,846	113,129
Minimum percentage of paid-in-capital required to be maintained under District of Columbia cooperative law				50.00%			50.00%
Cash flow hedging
Stockholder's Equity [Line Items]
Expected reclassification from AOCI over 12 months (less than)				$ 1,000
NCSC
Stockholder's Equity [Line Items]
Percentage of ownership by parent				100.00%			100.00%
Rural Telephone Finance Cooperative
Stockholder's Equity [Line Items]
Percentage of ownership by minority owners				100.00%			100.00%
CFC
Stockholder's Equity [Line Items]
General reserve required to be maintained as a percentage of membership fees collected				50.00%			50.00%
Number of additional board-approved reserves | reserve				1			1
NCSC
Stockholder's Equity [Line Items]
Minimum percentage of paid-in-capital required to be maintained under District of Columbia cooperative law				50.00%			50.00%
Total CFC Equity
Stockholder's Equity [Line Items]
Total equity	3,082,477			$ 3,290,524	3,082,477	2,991,462	$ 3,290,524	2,562,059
Net income (loss)				262,112	139,733	553,349
Patronage capital retirement				$ 52,978	46,846	113,129
Cooperative educational fund | CFC | Minimum
Stockholder's Equity [Line Items]
Minimum percentage of net earnings to be allocated to cooperative education fund as per bylaws of the entity				0.25%			0.25%
Cooperative educational fund | NCSC | Minimum
Stockholder's Equity [Line Items]
Minimum percentage of net earnings to be allocated to cooperative education fund as per bylaws of the entity				0.25%			0.25%
Patronage Capital Allocated
Stockholder's Equity [Line Items]
Total equity	948,526			$ 966,253	948,526	928,232	$ 966,253	1,006,115
Net income (loss)				71,517	67,140	60,599
Patronage capital retirement				$ 52,978	46,846	138,482
Patronage Capital Allocated | CFC
Stockholder's Equity [Line Items]
Net income (loss)	$ 67,000
Percentage of prior year's allocated patronage capital required to be retired				50.00%
Percentage of prior year's allocated patronage capital required to be held				50.00%
Period for which prior year's allocated patronage capital is required to be held				25 years
Retirement of allocated net earnings, percentage	50.00%
Patronage Capital Allocated | CFC | Subsequent event
Stockholder's Equity [Line Items]
Net income (loss)		$ 72,000
Total membership fees and educational fund
Stockholder's Equity [Line Items]
Total equity	$ 3,624			$ 3,769	3,624	3,576	3,769	3,534
Net income (loss)				1,100	1,100	1,100
Total membership fees and educational fund | CFC
Stockholder's Equity [Line Items]
Net income (loss)	1,000						1,000
Members’ Capital Reserve
Stockholder's Equity [Line Items]
Total equity	1,631,609			1,804,161	1,631,609	1,455,564	$ 1,804,161	$ 1,202,152
Net income (loss)				$ 171,739	$ 176,045	228,059
Patronage capital retirement						$ (25,353)
Members’ Capital Reserve | CFC
Stockholder's Equity [Line Items]
Net income (loss)	176,000
Members’ Capital Reserve | CFC | Subsequent event
Stockholder's Equity [Line Items]
Net income (loss)		172,000
Retained Earnings, Appropriated Patronage Capital Allocated and Retired | CFC
Stockholder's Equity [Line Items]
Net income (loss)	53,000		$ 53,000
Retained Earnings, Appropriated Patronage Capital Allocated and Retired | CFC | Subsequent event
Stockholder's Equity [Line Items]
Net income (loss)		62,000
Retained earnings allocation of fifty percent of prior year patronage capital allocation | CFC
Stockholder's Equity [Line Items]
Net income (loss)	34,000
Retained earnings allocation of fifty percent of prior year patronage capital allocation | CFC | Subsequent event
Stockholder's Equity [Line Items]
Net income (loss)		$ 36,000
Retirement of allocated net earnings, percentage		50.00%
Retained earnings prior year allocation held for 25 years | CFC
Stockholder's Equity [Line Items]
Net income (loss)	$ 19,000
Period for which prior year's allocated patronage capital is required to be held	25 years
Retained earnings prior year allocation held for 25 years | CFC | Subsequent event
Stockholder's Equity [Line Items]
Net income (loss)		$ 26,000
Period for which prior year's allocated patronage capital is required to be held		25 years