Other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Before-Tax Amount
Other comprehensive income (loss)	$ 10,241	$ (7,941)	$ 27,400	$ (20,669)
Tax Expense (Benefit)
Other comprehensive income (loss)	2,345	(1,244)	5,743	(4,125)
Net-of-Tax Amount
Other comprehensive income (loss)	7,896	(6,697)	21,657	(16,544)
Net unrealized (losses) gains on investments
Before-Tax Amount
Change in net unrealized gains and losses on investments	6,800	(3,074)	18,206	(13,208)
Less: reclassification adjustment for net losses (gains) included in net income	63	(292)	268	(607)
Other comprehensive income (loss)	6,863	(3,366)	18,474	(13,815)
Tax Expense (Benefit)
Change in net unrealized gains and losses on investments	1,429	(646)	3,824	(2,774)
Less: reclassification adjustment for net losses (gains) included in net income	13	(61)	56	(127)
Other comprehensive income (loss)	1,442	(707)	3,880	(2,901)
Net-of-Tax Amount
Change in net unrealized gains and losses on investments	5,371	(2,428)	14,382	(10,434)
Less: reclassification adjustment for net losses (gains) included in net income	50	(231)	212	(480)
Other comprehensive income (loss)	5,421	(2,659)	14,594	(10,914)
Foreign currency translation adjustments
Before-Tax Amount
Other comprehensive income (loss)	3,378	(4,575)	8,926	(6,854)
Tax Expense (Benefit)
Other comprehensive income (loss)	903	(537)	1,863	(1,224)
Net-of-Tax Amount
Other comprehensive income (loss)	$ 2,475	$ (4,038)	$ 7,063	$ (5,630)