Other comprehensive income (loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Before-Tax Amount
Other comprehensive income (loss)	$ 10,558	$ (4,781)	$ (24,280)
Tax Expense (Benefit)
Other comprehensive income (loss)	2,524	143	(7,768)
Net-of-Tax Amount
Other comprehensive income (loss)	8,034	(4,924)	(16,512)
Foreign currency translation adjustments
Before-Tax Amount
Other comprehensive income (loss)	11,050	(2,385)	(16,022)
Tax Expense (Benefit)
Other comprehensive income (loss)	2,696	982	(4,877)
Net-of-Tax Amount
Other comprehensive income (loss)	8,354	(3,367)	(11,145)
Unrealized net gains on investments
Before-Tax Amount
Other comprehensive income (loss), before reclassifications	2,718	544	(5,757)
Less: reclassification adjustment for net gains included in net income (loss)	(3,210)	(2,940)	(2,501)
Other comprehensive income (loss)	(492)	(2,396)	(8,258)
Tax Expense (Benefit)
Other comprehensive income (loss), before reclassifications	952	190	(2,016)
Less: reclassification adjustment for net gains included in net income (loss)	(1,124)	(1,029)	(875)
Other comprehensive income (loss)	(172)	(839)	(2,891)
Net-of-Tax Amount
Other comprehensive income (loss), before reclassifications	1,766	354	(3,741)
Less: reclassification adjustment for net gains included in net income (loss)	(2,086)	(1,911)	(1,626)
Other comprehensive income (loss)	$ (320)	$ (1,557)	$ (5,367)