Fair Value of Financial Instruments - Reconciliation Fair Value, Assets (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Accretion of discount (amortization of premium)	$ 4,484	$ 5,425
Total
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	1,594,205	1,621,482
Accretion of discount (amortization of premium)	3,452	5,234
Purchases	65,563	256,488
Paid-in-kind interest	14,174	18,268
Sales and repayments	(354,036)	(421,859)
Transfers into Level 3	810	61,541
Transfers out of Level 3	0	0
Fair value at end of period	1,177,359	1,702,694
The amount of total gains or losses for the period included in changes in net assets attributable to the change in unrealized gains or losses relating to investments still held at the reporting date	(135,242)	93,609
Total | Net realized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	(49,602)	27,008
Total | Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	(97,207)	134,532
Senior Secured Loans—First Lien
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	443,245	414,075
Accretion of discount (amortization of premium)	1,369	1,426
Purchases	65,317	110,486
Paid-in-kind interest	9,834	9,627
Sales and repayments	(76,948)	(122,163)
Transfers into Level 3	0	58,705
Transfers out of Level 3	0	0
Fair value at end of period	392,383	486,018
The amount of total gains or losses for the period included in changes in net assets attributable to the change in unrealized gains or losses relating to investments still held at the reporting date	(50,106)	16,286
Senior Secured Loans—First Lien | Net realized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	(2,864)	(12,186)
Senior Secured Loans—First Lien | Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	(47,570)	26,048
Senior Secured Loans—Second Lien
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	143,270	84,083
Accretion of discount (amortization of premium)	710	194
Purchases	0	110,150
Paid-in-kind interest	0	537
Sales and repayments	(73,544)	(49,577)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fair value at end of period	69,599	145,632
The amount of total gains or losses for the period included in changes in net assets attributable to the change in unrealized gains or losses relating to investments still held at the reporting date	(591)	22
Senior Secured Loans—Second Lien | Net realized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	(52)	446
Senior Secured Loans—Second Lien | Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	(785)	(201)
Senior Secured Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	10,074	10,371
Accretion of discount (amortization of premium)	38	36
Purchases	0	0
Paid-in-kind interest	0	0
Sales and repayments	(107)	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fair value at end of period	9,780	10,195
The amount of total gains or losses for the period included in changes in net assets attributable to the change in unrealized gains or losses relating to investments still held at the reporting date	(229)	(212)
Senior Secured Bonds | Net realized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	4	0
Senior Secured Bonds | Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	(229)	(212)
Unsecured Debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	54,374	104,659
Accretion of discount (amortization of premium)	105	105
Purchases	0	19,800
Paid-in-kind interest	4,340	7,916
Sales and repayments	(1,500)	(84,415)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fair value at end of period	57,743	54,918
The amount of total gains or losses for the period included in changes in net assets attributable to the change in unrealized gains or losses relating to investments still held at the reporting date	416	(293)
Unsecured Debt | Net realized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	8	(27,541)
Unsecured Debt | Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	416	34,394
Preferred Equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	400,414	497,288
Accretion of discount (amortization of premium)	1,230	3,473
Purchases	0	0
Paid-in-kind interest	0	188
Sales and repayments	(162,115)	(61,186)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fair value at end of period	259,205	431,651
The amount of total gains or losses for the period included in changes in net assets attributable to the change in unrealized gains or losses relating to investments still held at the reporting date	25,880	(6,787)
Preferred Equity | Net realized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	(11,847)	1,329
Preferred Equity | Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	31,523	(9,441)
Sustainable Infrastructure Investments, LLC
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	51,098	50,770
Accretion of discount (amortization of premium)	0	0
Purchases	0	0
Paid-in-kind interest	0	0
Sales and repayments	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fair value at end of period	49,192	52,047
The amount of total gains or losses for the period included in changes in net assets attributable to the change in unrealized gains or losses relating to investments still held at the reporting date	(1,906)	1,277
Sustainable Infrastructure Investments, LLC | Net realized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	0	0
Sustainable Infrastructure Investments, LLC | Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	(1,906)	1,277
Equity/Other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	491,730	460,236
Accretion of discount (amortization of premium)	0	0
Purchases	246	16,052
Paid-in-kind interest	0	0
Sales and repayments	(39,822)	(104,518)
Transfers into Level 3	810	2,836
Transfers out of Level 3	0	0
Fair value at end of period	339,457	522,233
The amount of total gains or losses for the period included in changes in net assets attributable to the change in unrealized gains or losses relating to investments still held at the reporting date	(108,706)	83,316
Equity/Other | Net realized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	(34,851)	64,960
Equity/Other | Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized gain (loss)	$ (78,656)	$ 82,667