Other comprehensive (loss) income - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Equity [Abstract]
Foreign currency translation adjustments, before-tax amount	$ 2,514	$ 6,037	$ (6,428)	$ 1,898
Foreign currency translation adjustments, tax	271	1,908	(2,172)	177
Foreign currency translation adjustments, net-of-tax amount	2,243	4,129	(4,256)	1,721
Change in unrealized (losses) gains on investments, before-tax amount	(10,308)	5,702	(6,063)	12,103
Change in unrealized (losses) gains on investments, tax	(3,608)	1,996	(2,122)	4,236
Change in unrealized (losses) gains on investments, net-of-tax amount	(6,700)	3,706	(3,941)	7,867
Less: reclassification adjustment for net gains included in net income, before-tax amount	(201)	(96)	(1,203)	(298)
Less: reclassification adjustment for net gains included in net income, tax	(70)	(34)	(421)	(105)
Less: reclassification adjustment for net gains included in net income, net-of-tax amount	(131)	(62)	(782)	(193)
Net unrealized (losses) gains, before-tax amount	(10,509)	5,606	(7,266)	11,805
Net unrealized (losses) gains, tax	(3,678)	1,962	(2,543)	4,131
Net unrealized (losses) gains, net-of-tax amount	(6,831)	3,644	(4,723)	7,674
Other comprehensive (loss) income, before taxes	(7,995)	11,643	(13,694)	13,703
Other comprehensive (loss) income, tax	(3,407)	3,870	(4,715)	4,308
Other comprehensive (loss) income, net-of-tax amount	$ (4,588)	$ 7,773	$ (8,979)	$ 9,395