Investment Portfolio - Operations Information for SIIJV (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Schedule of Investments [Line Items]
Total investment income		$ 30,443	$ 40,788	$ 115,014	$ 123,770
Interest expense	[1]	738	14,104	19,066	40,994
Administrative services		1,887	1,695	4,566	4,605
Custodian and accounting fees		820	768	2,365	2,216
Professional services		82	369	444	735
Net expenses		12,891	29,081	57,088	82,385
Net investment income before taxes		17,552	11,707	57,926	41,385
Net realized and unrealized gain (loss)		(44,141)	(62,791)	(140,404)	113,546
Sustainable Infrastructure Investments, LLC
Schedule of Investments [Line Items]
Total investment income		5,272	3,313	15,083	7,386
Interest expense		3,410	1,201	9,870	3,352
Administrative services		46	42	134	123
Custodian and accounting fees		58	23	161	127
Professional services		70	25	170	76
Other		11	44	31	44
Net expenses		3,595	1,335	10,366	3,722
Net investment income before taxes		1,677	1,978	4,717	3,664
Net realized and unrealized gain (loss)		(2,273)	836	(937)	(1,180)
Net increase (decrease) in net assets resulting from operations		$ (596)	$ 2,814	$ 3,780	$ 2,484

[1] See Note 9 for a discussion of the Company's financing arrangements.