Other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Before-Tax Amount
Other comprehensive income (loss)	$ 17,159	$ (12,728)
Tax Expense (Benefit)
Other comprehensive income (loss)	3,398	(2,881)
Net-of-Tax Amount
Other comprehensive income (loss)	13,761	(9,847)
Net unrealized (losses) gains on investments
Before-Tax Amount
Change in net unrealized gains and losses on investments	11,406	(10,134)
Less: reclassification adjustment for net losses (gains) included in net income	205	(315)
Other comprehensive income (loss)	11,611	(10,449)
Tax Expense (Benefit)
Change in net unrealized gains and losses on investments	2,395	(2,128)
Less: reclassification adjustment for net losses (gains) included in net income	43	(66)
Other comprehensive income (loss)	2,438	(2,194)
Net-of-Tax Amount
Change in net unrealized gains and losses on investments	9,011	(8,006)
Less: reclassification adjustment for net losses (gains) included in net income	162	(249)
Other comprehensive income (loss)	9,173	(8,255)
Foreign currency translation adjustments
Before-Tax Amount
Other comprehensive income (loss)	5,548	(2,279)
Tax Expense (Benefit)
Other comprehensive income (loss)	960	(687)
Net-of-Tax Amount
Other comprehensive income (loss)	$ 4,588	$ (1,592)