Investments in debt and equity securities - Additional Information (Details) $ in Thousands	Dec. 31, 2023 USD ($) investment	Dec. 31, 2022 USD ($)
Debt Securities, Available-for-sale [Line Items]
Net unrealized investment gains on equity securities	$ 11,200	$ 19,200
Debt securities	$ 610,236	611,934
Number of investments in an unrealized loss position | investment	321
Number of investments in an unrealized loss positions for more than 12 months | investment	261
Foreign
Debt Securities, Available-for-sale [Line Items]
Debt securities	$ 323,391	299,137
Foreign | Canada
Debt Securities, Available-for-sale [Line Items]
Debt securities	282,400	261,800
Foreign | United Kingdom
Debt Securities, Available-for-sale [Line Items]
Debt securities	$ 31,700	$ 28,000