Other comprehensive (loss) earnings - Additional Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Equity [Abstract]
Foreign currency translation adjustments, before-tax amount	$ (8,257)	$ 4,587	$ (6,359)	$ (5,247)
Foreign currency translation adjustments, tax	(2,227)	1,027	(2,051)	(1,605)
Foreign currency translation adjustments, net-of-tax amount	(6,030)	3,560	(4,308)	(3,642)
Change in unrealized (losses) gains on investments, before-tax amount	(1,487)	1,981	10,616	(10,851)
Change in unrealized (losses) gains on investments, tax	(521)	694	3,716	(3,798)
Change in unrealized (losses) gains on investments, net-of-tax amount	(966)	1,287	6,900	(7,053)
Less: reclassification adjustment for gains included in net earnings, before-tax amount	(283)	(520)	(581)	(2,421)
Less: reclassification adjustment for gains included in net earnings, tax	(98)	(182)	(203)	(847)
Less: reclassification adjustment for gains included in net earnings, net-of-tax amount	(185)	(338)	(378)	(1,574)
Net unrealized (losses) gains, before-tax amount	(1,770)	1,461	10,035	(13,272)
Net unrealized (losses) gains, tax	(619)	512	3,513	(4,645)
Net unrealized (losses) gains, net-of-tax amount	(1,151)	949	6,522	(8,627)
Other comprehensive (loss) earnings, before taxes	(10,027)	6,048	3,676	(18,519)
Other comprehensive (loss) earnings, tax	(2,846)	1,539	1,462	(6,250)
Other comprehensive (loss) earnings, net-of-tax amount	$ (7,181)	$ 4,509	$ 2,214	$ (12,269)