Debt - Schedule of Senior Securities (Including Debt Securities and Other Indebtedness) (Details) - Senior Notes - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
8.25% Notes due 2020
Debt Instrument [Line Items]
Total Amount Outstanding								$ 33,646
Asset Coverage Ratio Per Unit								$ 6,168
Average Market Value Per Unit								$ 1.02
GECCL Notes
Debt Instrument [Line Items]
Total Amount Outstanding				$ 30,293	$ 32,631	$ 32,631	$ 32,631
Asset Coverage Ratio Per Unit				$ 1,671	$ 1,701	$ 2,393	$ 5,010
Average Market Value Per Unit				$ 0.89	$ 1.01	$ 1.01	$ 1.02
GECCM Notes
Debt Instrument [Line Items]
Total Amount Outstanding	$ 45,610	$ 45,610	$ 45,610	$ 45,610	$ 46,398	$ 46,398
Asset Coverage Ratio Per Unit	$ 1,684	$ 1,544	$ 1,511	$ 1,671	$ 1,701	$ 2,393
Average Market Value Per Unit	$ 0.98	$ 0.99	$ 1.00	$ 0.84	$ 1.01	$ 0.98
GECCN Notes
Debt Instrument [Line Items]
Total Amount Outstanding		$ 42,823	$ 42,823	$ 42,823	$ 45,000
Asset Coverage Ratio Per Unit		$ 1,544	$ 1,511	$ 1,671	$ 1,701
Average Market Value Per Unit		$ 1.00	$ 1.00	$ 0.84	$ 1.00
GECCO Notes
Debt Instrument [Line Items]
Total Amount Outstanding	$ 57,500	$ 57,500	$ 57,500
Asset Coverage Ratio Per Unit	$ 1,684	$ 1,544	$ 1,511
Average Market Value Per Unit	$ 0.96	$ 1.00	$ 1.02
GECCZ Notes
Debt Instrument [Line Items]
Total Amount Outstanding	$ 40,000
Asset Coverage Ratio Per Unit	$ 1,684
Average Market Value Per Unit	$ 0.96
Revolving Credit Facility
Debt Instrument [Line Items]
Total Amount Outstanding		$ 10,000
Asset Coverage Ratio Per Unit		$ 1,544