Income taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Effective tax rates reconciliation
Statutory federal income tax rate (as a percent)			21.00%		21.00%	35.00%
Annual income tax rate (as a percent)	34.60%		34.60%	41.60%
Expense related to foreign tax credit utilization	$ 800
Tax benefit related to hurricane relief credits and an increase to foreign tax credit utilization		$ 500
Income tax expense	$ 3,743	$ 3,229	$ 9,942	$ 4,490