Other comprehensive (loss) income - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Equity [Abstract]
Foreign currency translation adjustments, before-tax amount	$ (5,787)	$ (8,257)	$ (12,215)	$ (6,359)
Foreign currency translation adjustments, tax	(1,505)	(2,227)	(3,677)	(2,051)
Foreign currency translation adjustments, net-of-tax amount	(4,282)	(6,030)	(8,538)	(4,308)
Change in unrealized (losses) gains on investments, before-tax amount	(239)	(1,487)	(6,303)	10,616
Change in unrealized (losses) gains on investments, tax	(84)	(521)	(2,207)	3,716
Change in unrealized (losses) gains on investments, net-of-tax amount	(155)	(966)	(4,096)	6,900
Less: reclassification adjustment for net gains included in net income, before-tax amount	(675)	(283)	(1,878)	(581)
Less: reclassification adjustment for net gains included in net income, tax	(236)	(98)	(657)	(203)
Less: reclassification adjustment for net gains included in net income, net-of-tax amount	(439)	(185)	(1,221)	(378)
Net unrealized (losses) gains, before-tax amount	(914)	(1,770)	(8,181)	10,035
Net unrealized (losses) gains, tax	(320)	(619)	(2,864)	3,513
Net unrealized (losses) gains, net-of-tax amount	(594)	(1,151)	(5,317)	6,522
Other comprehensive (loss) income, before taxes	(6,701)	(10,027)	(20,396)	3,676
Other comprehensive (loss) income, tax	(1,825)	(2,846)	(6,541)	1,462
Other comprehensive (loss) income, net-of-tax amount	$ (4,876)	$ (7,181)	$ (13,855)	$ 2,214