Other comprehensive (loss) income - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Equity [Abstract]
Foreign currency translation adjustments, before-tax amount	$ (16,022)	$ (10,405)	$ (10,349)
Foreign currency translation adjustments, tax	(4,877)	(2,840)	(3,530)
Foreign currency translation adjustments, net-of-tax amount	(11,145)	(7,565)	(6,819)
Change in unrealized gains (losses) on investments, before-tax amount	(5,757)	15,066	(11,203)
Change in unrealized gains (losses) on investments, tax	(2,016)	5,273	(3,921)
Change in unrealized gains (losses) on investments, net-of-tax amount	(3,741)	9,793	(7,282)
Less: reclassification adjustment for gains included in net earnings, before-tax amount	(2,501)	(853)	(2,463)
Less: reclassification adjustment for gains included in net earnings, tax	(875)	(298)	(862)
Less: reclassification adjustment for gains included in net earnings, net-of-tax amount	(1,626)	(555)	(1,601)
Net unrealized gains (losses), before-tax amount	(8,258)	14,213	(13,666)
Net unrealized gains (losses), tax	(2,891)	4,975	(4,783)
Net unrealized gains (losses), net-of-tax amount	(5,367)	9,238	(8,883)
Other comprehensive income (loss), before-tax amount	(24,280)	3,808	(24,015)
Other comprehensive income (loss), tax	(7,768)	2,135	(8,313)
Other comprehensive (loss) income, net of taxes	$ (16,512)	$ 1,673	$ (15,702)