Fair value of assets and liabilities - Interest Rate Sensitivity of Level 3 Valuations (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Dec. 31, 2025
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Increase in income and net assets due to decrease in interest rate	$ 321,000	$ 491,100
Interest rate 100bps
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Increase in interest rate (percent)	1.00%	1.00%
Decrease in interest rate (percent)	1.00%	1.00%
Decrease in income and net assets due to increase in interest rate	$ (118,337)	$ (166,466)
Increase in income and net assets due to decrease in interest rate	$ 128,098	$ 175,812
Interest rate 50bps
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Increase in interest rate (percent)	0.50%	0.50%
Decrease in interest rate (percent)	0.50%	0.50%
Decrease in income and net assets due to increase in interest rate	$ (59,576)	$ (83,662)
Increase in income and net assets due to decrease in interest rate	$ 63,554	$ 87,423