Share-based compensation - 2011 Director Plan (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-based compensation
Income tax benefit	$ 500	$ 400	$ 1,000	$ 700
Common stock
Share-based compensation
Shares granted (in shares)	29,320	35,632	29,320	36,100
Fair value	$ 1,200	$ 1,300	$ 1,200	$ 1,300
Income tax benefit	$ 300	$ 300	$ 300	$ 300