Investments - Composition of the Company's investment portfolio by industry, at amortized cost and fair value (with corresponding percentage of total portfolio investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Investments
Amortized Cost	$ 1,535,660	$ 1,465,352
Fair Value	$ 1,543,910	$ 1,468,996
Amortized Cost, Percentage of total portfolio investments	100.00%	100.00%
Fair Value, Percentage of total portfolio investments	100.00%	100.00%
Aerospace & Defense
Investments
Amortized Cost	$ 21,446	$ 21,418
Fair Value	$ 20,974	$ 21,049
Amortized Cost, Percentage of total portfolio investments	1.40%	1.50%
Fair Value, Percentage of total portfolio investments	1.30%	1.40%
Automotive
Investments
Amortized Cost	$ 39,434	$ 39,131
Fair Value	$ 38,482	$ 38,843
Amortized Cost, Percentage of total portfolio investments	2.60%	2.70%
Fair Value, Percentage of total portfolio investments	2.50%	2.70%
Banking
Investments
Amortized Cost	$ 35,594	$ 35,534
Fair Value	$ 37,064	$ 37,979
Amortized Cost, Percentage of total portfolio investments	2.30%	2.40%
Fair Value, Percentage of total portfolio investments	2.40%	2.60%
Beverage, Food & Tobacco
Investments
Amortized Cost	$ 16,584	$ 16,807
Fair Value	$ 16,212	$ 16,439
Amortized Cost, Percentage of total portfolio investments	1.10%	1.10%
Fair Value, Percentage of total portfolio investments	1.00%	1.10%
Capital Equipment
Investments
Amortized Cost	$ 54,893	$ 55,062
Fair Value	$ 55,272	$ 56,074
Amortized Cost, Percentage of total portfolio investments	3.60%	3.70%
Fair Value, Percentage of total portfolio investments	3.60%	3.80%
Construction & Building
Investments
Amortized Cost	$ 35,102	$ 34,932
Fair Value	$ 35,122	$ 34,877
Amortized Cost, Percentage of total portfolio investments	2.30%	2.40%
Fair Value, Percentage of total portfolio investments	2.30%	2.40%
Consumer Goods: Durable
Investments
Amortized Cost	$ 41,730	$ 42,278
Fair Value	$ 39,552	$ 40,357
Amortized Cost, Percentage of total portfolio investments	2.70%	2.90%
Fair Value, Percentage of total portfolio investments	2.50%	2.70%
Consumer Goods: Non-Durable
Investments
Amortized Cost	$ 35,617	$ 32,697
Fair Value	$ 35,671	$ 32,843
Amortized Cost, Percentage of total portfolio investments	2.30%	2.20%
Fair Value, Percentage of total portfolio investments	2.30%	2.20%
Containers, Packaging & Glass
Investments
Amortized Cost	$ 11,663	$ 11,674
Fair Value	$ 11,885	$ 11,675
Amortized Cost, Percentage of total portfolio investments	0.70%	0.80%
Fair Value, Percentage of total portfolio investments	0.80%	0.80%
Energy: Oil & Gas
Investments
Amortized Cost	$ 3,092	$ 3,970
Fair Value	$ 2,778	$ 3,597
Amortized Cost, Percentage of total portfolio investments	0.20%	0.30%
Fair Value, Percentage of total portfolio investments	0.20%	0.20%
Environmental Industries
Investments
Amortized Cost	$ 34,900	$ 30,612
Fair Value	$ 35,801	$ 31,457
Amortized Cost, Percentage of total portfolio investments	2.30%	2.10%
Fair Value, Percentage of total portfolio investments	2.30%	2.10%
FIRE: Finance
Investments
Amortized Cost	$ 69,875	$ 67,608
Fair Value	$ 68,987	$ 66,639
Amortized Cost, Percentage of total portfolio investments	4.50%	4.60%
Fair Value, Percentage of total portfolio investments	4.50%	4.50%
FIRE: Insurance
Investments
Amortized Cost	$ 16,318	$ 9,709
Fair Value	$ 16,019	$ 9,641
Amortized Cost, Percentage of total portfolio investments	1.10%	0.70%
Fair Value, Percentage of total portfolio investments	1.00%	0.70%
FIRE: Real Estate
Investments
Amortized Cost	$ 80,090	$ 69,695
Fair Value	$ 82,196	$ 71,154
Amortized Cost, Percentage of total portfolio investments	5.20%	4.70%
Fair Value, Percentage of total portfolio investments	5.30%	4.80%
Healthcare & Pharmaceuticals
Investments
Amortized Cost	$ 227,471	$ 209,460
Fair Value	$ 229,611	$ 210,831
Amortized Cost, Percentage of total portfolio investments	14.80%	14.30%
Fair Value, Percentage of total portfolio investments	14.90%	14.40%
High Tech Industries
Investments
Amortized Cost	$ 185,309	$ 180,078
Fair Value	$ 186,220	$ 180,823
Amortized Cost, Percentage of total portfolio investments	12.10%	12.30%
Fair Value, Percentage of total portfolio investments	12.10%	12.30%
Hotels, Gaming & Leisure
Investments
Amortized Cost	$ 2,491	$ 2,316
Fair Value	$ 2,484	$ 2,331
Amortized Cost, Percentage of total portfolio investments	0.20%	0.10%
Fair Value, Percentage of total portfolio investments	0.20%	0.20%
Media: Advertising, Printing & Publishing
Investments
Amortized Cost	$ 135,724	$ 128,704
Fair Value	$ 137,498	$ 129,362
Amortized Cost, Percentage of total portfolio investments	8.80%	8.80%
Fair Value, Percentage of total portfolio investments	8.90%	8.80%
Media: Broadcasting & Subscription
Investments
Amortized Cost	$ 2,415	$ 2,059
Fair Value	$ 2,376	$ 2,019
Amortized Cost, Percentage of total portfolio investments	0.10%	0.10%
Fair Value, Percentage of total portfolio investments	0.10%	0.10%
Media: Diversified & Production
Investments
Amortized Cost	$ 45,061	$ 46,881
Fair Value	$ 44,527	$ 46,348
Amortized Cost, Percentage of total portfolio investments	2.90%	3.20%
Fair Value, Percentage of total portfolio investments	2.90%	3.20%
Retail
Investments
Amortized Cost	$ 930
Fair Value	$ 930
Amortized Cost, Percentage of total portfolio investments	0.10%
Fair Value, Percentage of total portfolio investments	0.10%
Services: Business
Investments
Amortized Cost	$ 192,177	$ 184,567
Fair Value	$ 194,301	$ 184,535
Amortized Cost, Percentage of total portfolio investments	12.50%	12.60%
Fair Value, Percentage of total portfolio investments	12.60%	12.60%
Services: Consumer
Investments
Amortized Cost	$ 84,194	$ 77,065
Fair Value	$ 85,021	$ 77,998
Amortized Cost, Percentage of total portfolio investments	5.50%	5.30%
Fair Value, Percentage of total portfolio investments	5.50%	5.30%
Telecommunications
Investments
Amortized Cost	$ 36,274	$ 36,508
Fair Value	$ 36,473	$ 36,415
Amortized Cost, Percentage of total portfolio investments	2.40%	2.50%
Fair Value, Percentage of total portfolio investments	2.40%	2.50%
Transportation: Cargo
Investments
Amortized Cost	$ 98,134	$ 97,803
Fair Value	$ 98,663	$ 97,153
Amortized Cost, Percentage of total portfolio investments	6.40%	6.70%
Fair Value, Percentage of total portfolio investments	6.40%	6.60%
Wholesale
Investments
Amortized Cost	$ 29,142	$ 28,784
Fair Value	$ 29,791	$ 28,557
Amortized Cost, Percentage of total portfolio investments	1.90%	2.00%
Fair Value, Percentage of total portfolio investments	1.90%	2.00%