Retirement benefits (Additional Information) (Details) - person	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Matched employee contribution (as a percent)	100.00%	100.00%	100.00%
Employee contribution on gross wages (as a percent)	7.00%	7.00%	7.00%
Number of employee participating in plan	122	173	210

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

No definition available.

Name:	us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration