Other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Before-Tax Amount
Change in net unrealized gains on investments	$ 548	$ 9,511	$ 4,343	$ 18,417
Less: reclassification adjustment for net gains included in net income	(145)	(178)	(710)	(276)
Net unrealized gains	403	9,333	3,633	18,141
Foreign currency translation adjustments	4,334	(1,413)	6,014	3,944
Other comprehensive income	4,737	7,920	9,647	22,085
Tax Expense (Benefit)
Change in net unrealized gains on investments	193	3,329	1,521	6,446
Less: reclassification adjustment for net gains included in net income	(51)	(63)	(249)	(97)
Net unrealized gains	142	3,266	1,272	6,349
Foreign currency translation adjustments	1,130	290	1,485	2,352
Other comprehensive income	1,272	3,556	2,757	8,701
Net-of-Tax Amount
Change in net unrealized gains on investments	355	6,182	2,822	11,971
Less: reclassification adjustment for net gains included in net income	(94)	(115)	(461)	(179)
Net unrealized gains	261	6,067	2,361	11,792
Foreign currency translation adjustments	3,204	(1,703)	4,529	1,592
Other comprehensive income	$ 3,465	$ 4,364	$ 6,890	$ 13,384