COMMITMENTS AND CONTINGENCIES (Schedule of Future Minimum Transportation Fees Payment) (Details) $ in Thousands	Dec. 31, 2019 USD ($)
COMMITMENTS AND CONTINGENCIES [Abstract]
2020	$ 747
2021	618
2022	412
Balance as of December 31, 2019	$ 1,777

Future minimum transportation fees payment Year Three.

No definition available.

Name:	hrst_FutureMinimumTransportationFeesPaymentYearThree
Namespace Prefix:	hrst_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant