Commitments and Contingencies - Schedule of Supplemental Balance Sheet Information Related to Leases (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Operating lease right-of-use assets	$ 314	$ 430
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssets	us-gaap:OtherAssets
Operating lease liabilities	$ 314	$ 430
Operating Lease, Liability, Statement of Financial Position [Extensible List]	amsf:AccountsPayableAndOtherLiabilities	amsf:AccountsPayableAndOtherLiabilities
Finance lease right-of-use assets	$ 365	$ 185
Finance lease accumulated amortization right-of-use assets	(185)	(179)
Property and equipment, net	$ 180	$ 6
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:PropertyPlantAndEquipmentNet	us-gaap:PropertyPlantAndEquipmentNet
Finance lease liabilities	$ 185	$ 54
Finance Lease, Liability, Statement of Financial Position [Extensible List]	amsf:AccountsPayableAndOtherLiabilities	amsf:AccountsPayableAndOtherLiabilities