Investment Portfolio - Investment Portfolio, at Fair Value and Percentage (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2023		Dec. 31, 2022
Schedule of Investments [Line Items]
Investments, at fair value	$ 1,742,661	[1],[2],[3]	$ 2,047,115	[4],[5]
Percentage of Portfolio | Industry Concentration Risk
Schedule of Investments [Line Items]
Percentage of Portfolio	100.00%		100.00%
Upstream
Schedule of Investments [Line Items]
Investments, at fair value	$ 652,180		$ 854,974
Upstream | Percentage of Portfolio | Industry Concentration Risk
Schedule of Investments [Line Items]
Percentage of Portfolio	37.00%		42.00%
Midstream
Schedule of Investments [Line Items]
Investments, at fair value	$ 582,167		$ 646,488
Midstream | Percentage of Portfolio | Industry Concentration Risk
Schedule of Investments [Line Items]
Percentage of Portfolio	33.00%		32.00%
Power
Schedule of Investments [Line Items]
Investments, at fair value	$ 348,009		$ 386,007
Power | Percentage of Portfolio | Industry Concentration Risk
Schedule of Investments [Line Items]
Percentage of Portfolio	20.00%		19.00%
Service & Equipment
Schedule of Investments [Line Items]
Investments, at fair value	$ 97,727		$ 99,171
Service & Equipment | Percentage of Portfolio | Industry Concentration Risk
Schedule of Investments [Line Items]
Percentage of Portfolio	6.00%		5.00%
Industrials
Schedule of Investments [Line Items]
Investments, at fair value	$ 9,641		$ 9,377
Industrials | Percentage of Portfolio | Industry Concentration Risk
Schedule of Investments [Line Items]
Percentage of Portfolio	1.00%		0.00%
Sustainable Infrastructure Investments, LLC
Schedule of Investments [Line Items]
Investments, at fair value	$ 52,937		$ 51,098
Sustainable Infrastructure Investments, LLC | Percentage of Portfolio | Industry Concentration Risk
Schedule of Investments [Line Items]
Percentage of Portfolio	3.00%		2.00%

[1] Security may be an obligation of one or more entities affiliated with the named company.

[2] Security or portion thereof is pledged as collateral supporting the amounts outstanding under the Senior Secured Notes with JPMorgan Chase Bank, N.A. (see Note 9).

[3] See Note 8 for additional information regarding the fair value of the Company’s financial instruments.

[4] Security may be an obligation of one or more entities affiliated with the named company.

[5] See Note 8 for additional information regarding the fair value of the Company’s financial instruments.