Other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Before-Tax Amount
Change in unrealized gains on investments, before-tax amount	$ 9,511	$ (10,308)	$ 18,417	$ (6,063)
Less: reclassification adjustment for net gains included in net income, before-tax amount	(178)	(201)	(276)	(1,203)
Net unrealized gains, before-tax amount	9,333	(10,509)	18,141	(7,266)
Foreign currency translation adjustments, before-tax amount	(1,413)	2,514	3,944	(6,428)
Other comprehensive income (loss), before taxes	7,920	(7,995)	22,085	(13,694)
Tax Expense (Benefit)
Change in unrealized gains on investments, tax expense (benefit)	3,329	(3,608)	6,446	(2,122)
Less: reclassification adjustment for net gains included in net income, tax expense (benefit)	(63)	(70)	(97)	(421)
Net unrealized gains, tax expense (benefit)	3,266	(3,678)	6,349	(2,543)
Foreign currency translation adjustments, tax expense (benefit)	290	271	2,352	(2,172)
Other comprehensive income (loss), tax expense (benefit)	3,556	(3,407)	8,701	(4,715)
Net-of-Tax Amount
Change in unrealized gains on investments, net-of-tax amount	6,182	(6,700)	11,971	(3,941)
Less: reclassification adjustment for net gains included in net income, net-of-tax amount	(115)	(131)	(179)	(782)
Net unrealized gains, net-of-tax amount	6,067	(6,831)	11,792	(4,723)
Foreign currency translation adjustments, net-of-tax amount	(1,703)	2,243	1,592	(4,256)
Other comprehensive income (loss), net-of-tax amount	$ 4,364	$ (4,588)	$ 13,384	$ (8,979)