Other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Before-Tax Amount
Change in net unrealized gains on investments	$ 95	$ (405)	$ 4,438	$ 18,012
Less: reclassification adjustment for net gains included in net income	(508)	(1,330)	(1,218)	(1,606)
Net unrealized gains	(413)	(1,735)	3,220	16,406
Foreign currency translation adjustments	5,817	(2,363)	11,831	1,581
Other comprehensive income (loss)	5,404	(4,098)	15,051	17,987
Tax Expense (Benefit)
Change in net unrealized gains on investments	32	(142)	1,553	6,304
Less: reclassification adjustment for net gains included in net income	(177)	(465)	(426)	(562)
Net unrealized gains	(145)	(607)	1,127	5,742
Foreign currency translation adjustments	1,676	(555)	3,161	1,797
Other comprehensive income (loss)	1,531	(1,162)	4,288	7,539
Net-of-Tax Amount
Change in net unrealized gains on investments	63	(263)	2,885	11,708
Less: reclassification adjustment for net gains included in net income	(331)	(865)	(792)	(1,044)
Net unrealized gains	(268)	(1,128)	2,093	10,664
Foreign currency translation adjustments	4,141	(1,808)	8,670	(216)
Other comprehensive income (loss)	$ 3,873	$ (2,936)	$ 10,763	$ 10,448