Leases - Schedule of Supplemental Balance Sheet Information Related to Leases (Detail) $ in Thousands	Jun. 30, 2019 USD ($)
Leases [Abstract]
Operating lease right-of-use assets	$ 501
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssets
Operating lease liabilities	$ 501
Operating Lease, Liability, Statement of Financial Position [Extensible List]	amsf:AccountsPayableAndOtherLiabilities
Finance lease right-of-use assets	$ 185
Finance lease accumulated amortization right-of-use assets	(149)
Property and equipment, net	$ 36
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:PropertyPlantAndEquipmentNet
Finance lease liabilities	$ 78
Finance Lease, Liability, Statement of Financial Position [Extensible List]	amsf:AccountsPayableAndOtherLiabilities