Fair value of assets and liabilities - Interest Rate Sensitivity of Level 3 Valuations (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Increase in income and net assets due to decrease in interest rate	$ 461,500	$ 466,300
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	$ (160,020)	$ (153,241)
Increase in income and net assets due to decrease in interest rate	$ 166,354	$ 159,169
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	$ (81,092)	$ (77,644)
Increase in income and net assets due to decrease in interest rate	$ 82,043	$ 78,514