Investment income and net realized and unrealized gains - Proceeds from Sale of Investment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total proceeds from sales of investments in securities	$ 37,240	$ 50,605	$ 49,442
Total proceeds from sales of investments in securities			49,442
Debt securities
Debt Securities, Available-for-sale [Line Items]
Total proceeds from sales of investments in securities	36,449	46,834
Total proceeds from sales of investments in securities			43,556
Equity securities
Debt Securities, Available-for-sale [Line Items]
Proceeds from sales of equity securities	$ 791	$ 3,771
Total proceeds from sales of investments in securities			$ 5,886