Other comprehensive (loss) income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Before-Tax Amount
Change in net unrealized gains and losses on investments	$ (3,074)	$ 548	$ (13,208)	$ 4,343
Less: reclassification adjustment for net gains included in net income	(292)	(145)	(607)	(710)
Net unrealized gains	(3,366)	403	(13,815)	3,633
Foreign currency translation adjustments	(4,575)	4,334	(6,854)	6,014
Other comprehensive (loss) income	(7,941)	4,737	(20,669)	9,647
Tax Expense (Benefit)
Change in net unrealized gains and losses on investments	(646)	193	(2,774)	1,521
Less: reclassification adjustment for net gains included in net income	(61)	(51)	(127)	(249)
Net unrealized gains	(707)	142	(2,901)	1,272
Foreign currency translation adjustments	(537)	1,130	(1,224)	1,485
Other comprehensive (loss) income	(1,244)	1,272	(4,125)	2,757
Net-of-Tax Amount
Change in net unrealized gains and losses on investments	(2,428)	355	(10,434)	2,822
Less: reclassification adjustment for net gains included in net income	(231)	(94)	(480)	(461)
Net unrealized gains	(2,659)	261	(10,914)	2,361
Foreign currency translation adjustments	(4,038)	3,204	(5,630)	4,529
Other comprehensive (loss) income	$ (6,697)	$ 3,465	$ (16,544)	$ 6,890