Fair Value of Financial Instruments - Schedule of Level 3 Assets measured at Fair Value (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 1,058,670	$ 1,020,800
Transfers in (out) of Level 3	9,181	(51)
Purchases of investments	91,887	168,089
PIK interest	11,449	9,582
Sales or prepayments of investments	(225,106)	(144,661)
Scheduled principal repayments of investments	(9,431)	(2,380)
Amortization of Fixed Income Premiums or Accretion of Discounts and ETP	3,344	8,278
Net realized gain (loss)	$ 6,243	$ 0
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Debt and Equity Securities, Realized Gain (Loss)	Debt and Equity Securities, Realized Gain (Loss)
Net change in unrealized gain (loss)	$ (23,057)	$ (5,448)
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Debt and Equity Securities, Unrealized Gain (Loss)	Debt and Equity Securities, Unrealized Gain (Loss)
Ending Balance	$ 923,180	$ 1,054,209
Net change in unrealized gain (loss) on Level 3 investments still held	$ (13,338)	$ (4,828)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Debt and Equity Securities, Unrealized Gain (Loss)	Debt and Equity Securities, Unrealized Gain (Loss)
Senior Secured Loans
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 950,092	$ 964,099
Transfers in (out) of Level 3	0	0
Purchases of investments	86,073	164,321
PIK interest	10,801	9,364
Sales or prepayments of investments	(186,751)	(144,575)
Scheduled principal repayments of investments	(9,431)	(2,380)
Amortization of Fixed Income Premiums or Accretion of Discounts and ETP	3,216	8,184
Net realized gain (loss)	(1,501)	0
Net change in unrealized gain (loss)	(4,225)	(9,628)
Ending Balance	848,274	989,385
Net change in unrealized gain (loss) on Level 3 investments still held	(2,650)	(9,007)
Second Lien Loans
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	20,152	14,399
Transfers in (out) of Level 3	0	0
Purchases of investments	0	0
PIK interest	648	218
Sales or prepayments of investments	0	0
Scheduled principal repayments of investments	0	0
Amortization of Fixed Income Premiums or Accretion of Discounts and ETP	128	94
Net realized gain (loss)	0	0
Net change in unrealized gain (loss)	249	(210)
Ending Balance	21,177	14,501
Net change in unrealized gain (loss) on Level 3 investments still held	248	(210)
Convertible Note
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	0	1,357
Transfers in (out) of Level 3	0	0
Purchases of investments	1,000	339
PIK interest	0	0
Sales or prepayments of investments	(1,000)	0
Scheduled principal repayments of investments	0	0
Amortization of Fixed Income Premiums or Accretion of Discounts and ETP	0	0
Net realized gain (loss)	0	0
Net change in unrealized gain (loss)	0	(1,696)
Ending Balance	0	0
Net change in unrealized gain (loss) on Level 3 investments still held	0	(1,696)
Preferred Stock/Units
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	73,460	26,285
Transfers in (out) of Level 3	9,181	0
Purchases of investments	2,773	0
PIK interest	0	0
Sales or prepayments of investments	(37,355)	0
Scheduled principal repayments of investments	0	0
Amortization of Fixed Income Premiums or Accretion of Discounts and ETP	0	0
Net realized gain (loss)	9,255	0
Net change in unrealized gain (loss)	(19,928)	6,387
Ending Balance	37,386	32,672
Net change in unrealized gain (loss) on Level 3 investments still held	(10,673)	6,387
Common Stock/Units
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	16	872
Transfers in (out) of Level 3	0	0
Purchases of investments	0	0
PIK interest	0	0
Sales or prepayments of investments	0	0
Scheduled principal repayments of investments	0	0
Amortization of Fixed Income Premiums or Accretion of Discounts and ETP	0	0
Net realized gain (loss)	0	0
Net change in unrealized gain (loss)	5	(753)
Ending Balance	21	119
Net change in unrealized gain (loss) on Level 3 investments still held	5	(753)
Equity Interest
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	1,198	950
Transfers in (out) of Level 3	0	0
Purchases of investments	0	0
PIK interest	0	0
Sales or prepayments of investments	0	0
Scheduled principal repayments of investments	0	0
Amortization of Fixed Income Premiums or Accretion of Discounts and ETP	0	0
Net realized gain (loss)	0	0
Net change in unrealized gain (loss)	(4)	248
Ending Balance	1,194	1,198
Net change in unrealized gain (loss) on Level 3 investments still held	(4)	248
Warrants
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	13,752	12,838
Transfers in (out) of Level 3	0	(51)
Purchases of investments	2,041	3,429
PIK interest	0	0
Sales or prepayments of investments	0	(86)
Scheduled principal repayments of investments	0	0
Amortization of Fixed Income Premiums or Accretion of Discounts and ETP	0	0
Net realized gain (loss)	(1,511)	0
Net change in unrealized gain (loss)	846	204
Ending Balance	15,128	16,334
Net change in unrealized gain (loss) on Level 3 investments still held	$ (264)	$ (203)

Runway Growth Finance Corp. 10-Q