Other comprehensive earnings (loss) - Additional Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity [Abstract]
Foreign currency translation adjustments, before-tax amount	$ (10,405)	$ (10,349)	$ 3,880
Foreign currency translation adjustments, tax	(2,840)	(3,530)	961
Foreign currency translation adjustments, net-of-tax amount	(7,565)	(6,819)	2,919
Change in unrealized gains (losses) on investments, before-tax amount	15,066	(11,203)	14,213
Change in unrealized gains (losses) on investments, tax	5,273	(3,921)	4,975
Change in unrealized (losses) gains on investments, net-of-tax amount	9,793	(7,282)	9,238
Less: reclassification adjustment for gains included in net earnings, before-tax amount	(853)	(2,463)	(3,470)
Less: reclassification adjustment for gains included in net earnings, tax	(298)	(862)	(1,215)
Less: reclassification adjustment for gains included in net earnings, net-of-tax amount	(555)	(1,601)	(2,255)
Net unrealized gains (losses), before-tax amount	14,213	(13,666)	10,743
Net unrealized gains (losses), tax	4,975	(4,783)	3,760
Net unrealized gains (losses), net-of-tax amount	9,238	(8,883)	6,983
Other comprehensive earnings (loss), before-tax amount	3,808	(24,015)	14,623
Other comprehensive earnings (loss), tax	2,135	(8,313)	4,721
Other comprehensive earnings (loss), net-of-tax amount	$ 1,673	$ (15,702)	$ 9,902