Other comprehensive (loss) earnings - Additional Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Equity [Abstract]
Foreign currency translation adjustments, before-tax amount	$ (8,942)	$ (4,140)
Foreign currency translation adjustments, tax	(2,443)	(1,732)
Foreign currency translation adjustments, net-of-tax amount	(6,499)	(2,408)
Change in unrealized gains on investments, before-tax amount	4,245	6,402
Change in unrealized gains on investments, tax	1,486	2,241
Change in unrealized gains on investments, net-of-tax amount	2,759	4,161
Less: reclassification adjustment for gains included in net earnings, before-tax amount	(1,002)	(202)
Less: reclassification adjustment for gains included in net earnings, tax	(351)	(71)
Less: reclassification adjustment for gains included in net earnings, net-of-tax amount	(651)	(131)
Net unrealized gains, before-tax amount	3,243	6,200
Net unrealized gains, tax	1,135	2,170
Net unrealized gains, net-of-tax amount	2,108	4,030
Other comprehensive (loss) earnings, before taxes	(5,699)	2,060
Other comprehensive (loss) earnings, tax	(1,308)	438
Other comprehensive (loss) earnings, net-of-tax amount	$ (4,391)	$ 1,622