Investment income and other gains and losses - Gross Realized Investment and Other Gains and Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Investment income	$ 18,925	$ 16,850	$ 16,806
Investment and other (losses) gains – net:
Realized gains	9,882	5,948	12,018
Realized losses	(10,548)	(7,317)	(5,274)
Investment and other (losses) gains – net	(666)	(1,369)	6,744
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Investment income	16,476	15,181	14,593
Short-term investments, cash equivalents and other
Schedule of Available-for-sale Securities [Line Items]
Investment income	$ 2,449	$ 1,669	$ 2,213