Investment and other gains (losses) - Gross Realized Investment and Other Gains and Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Securities, Available-for-sale [Line Items]
Investment income	$ 19,737	$ 18,932	$ 18,925
Investment and other gains (losses) – net:
Realized gains	2,698	4,997	9,882
Realized losses	(483)	(2,790)	(10,548)
Net unrealized investment losses recognized on equity securities still held	(2,162)	0	0
Investment and other gains (losses) – net	53	2,207	(666)
Debt securities
Debt Securities, Available-for-sale [Line Items]
Investment income	17,431	17,222	16,476
Short-term investments, cash equivalents and other
Debt Securities, Available-for-sale [Line Items]
Investment income	$ 2,306	$ 1,710	$ 2,449