Investment income and other gains and losses - Gross Realized Investment and Other Gains and Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Investment income	$ 18,932	$ 18,925	$ 16,850
Realized investment and other gains (losses) – net:
Realized gains	4,997	9,882	5,948
Realized losses	(2,790)	(10,548)	(7,317)
Investment and other (losses) gains – net	2,207	(666)	(1,369)
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Investment income	17,222	16,476	15,181
Short-term investments, cash equivalents and other
Schedule of Available-for-sale Securities [Line Items]
Investment income	$ 1,710	$ 2,449	$ 1,669