Significant Accounting Policies Self-Insurance Accruals (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Significant Accounting Policies [Abstract]
Health insurance reserves	$ 4.5	$ 6.0

Self insurance health insurance reserves for estimated cost of health related claims, including estimated expenses incurred but not reported

No definition available.

Name:	htld_HealthInsuranceReserves
Namespace Prefix:	htld_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant