Income taxes - Schedule of Allocation of Federal and State Income Taxes between Current and Deferred Portions (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Current	$ 13,123	$ 9,167	$ 37,038	$ 25,907
Provision for deferred income taxes	(15,163)	(1,449)	(31,543)	(5,900)
Total	$ (2,040)	$ 7,718	$ 5,495	$ 20,007