Income taxes - Schedule of Allocation of Federal and State Income Taxes between Current and Deferred Portions (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Tax Disclosure [Abstract]
Current	$ 15,747	$ 6,546	$ 23,915	$ 16,740
Provision for deferred income taxes	(8,292)	(232)	(16,380)	(4,451)
Total	$ 7,455	$ 6,314	$ 7,535	$ 12,289