Other comprehensive (loss) income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Before-Tax Amount
Other comprehensive (loss) income	$ (971)	$ 5,404	$ (21,640)	$ 15,051
Tax Expense (Benefit)
Other comprehensive (loss) income	(64)	1,531	(4,189)	4,288
Net-of-Tax Amount
Other comprehensive (loss) income	(907)	3,873	(17,451)	10,763
Net unrealized (losses) gains on investments
Before-Tax Amount
Change in net unrealized gains and losses on investments	(2,417)	95	(15,625)	4,438
Less: reclassification adjustment for net gains included in net income	(174)	(508)	(781)	(1,218)
Other comprehensive (loss) income	(2,591)	(413)	(16,406)	3,220
Tax Expense (Benefit)
Change in net unrealized gains and losses on investments	(507)	32	(3,281)	1,553
Less: reclassification adjustment for net gains included in net income	(37)	(177)	(164)	(426)
Other comprehensive (loss) income	(544)	(145)	(3,445)	1,127
Net-of-Tax Amount
Change in net unrealized gains and losses on investments	(1,910)	63	(12,344)	2,885
Less: reclassification adjustment for net gains included in net income	(137)	(331)	(617)	(792)
Other comprehensive (loss) income	(2,047)	(268)	(12,961)	2,093
Foreign currency translation adjustments
Before-Tax Amount
Other comprehensive (loss) income	1,620	5,817	(5,234)	11,831
Tax Expense (Benefit)
Other comprehensive (loss) income	480	1,676	(744)	3,161
Net-of-Tax Amount
Other comprehensive (loss) income	$ 1,140	$ 4,141	$ (4,490)	$ 8,670