Income taxes - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Reduction in deferred tax liability due to change in tax rate	$ 0	$ 5,894	$ 0
Deferred tax liability increased due to change in taxable status	0	0	13,181
Deferred income tax expense related to unrealized gain on available for sale securities	6,359	$ 6,458	$ 9,257
C Corporation [Member]
Income Taxes [Line Items]
Deferred income tax expense related to unrealized gain on available for sale securities	$ 2,955