Employee retirement plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan [Abstract]
Number of days of continuous service for eligibility to participate in defined contribution benefit plan	30 days
Maximum contribution percentage of employee salary	15.00%
Percentage of employee contribution matched by Board of Directors	50.00%
Maximum percentage of each participant’s compensation	8.00%	8.00%	8.00%
Employee eligibility period for matching contribution (years)	1 year
Company contributions	$ 9.6	$ 9.4	$ 9.5