Investment income and net realized and unrealized gains - Income from Investments and Net Realized and Unrealized Gains (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Investment income	$ 19,795	$ 19,737	$ 18,932
Net realized and unrealized gains:
Realized gains	53,465	2,698	4,997
Realized losses	(14,747)	(483)	(2,790)
Net unrealized investment gains (losses) recognized on equity securities still held	4,042	(2,162)	0
Investment and other gains (losses) – net	42,760	53	2,207
Debt securities
Debt Securities, Available-for-sale [Line Items]
Investment income	15,769	17,431	17,222
Short-term investments, cash equivalents and other
Debt Securities, Available-for-sale [Line Items]
Investment income	$ 4,026	$ 2,306	$ 1,710