Leases - Schedule of Supplemental Balance Sheet Information Related to Leases (Detail) $ in Thousands	Mar. 31, 2019 USD ($)
Leases [Abstract]
Operating lease right-of-use assets	$ 539
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssets
Operating lease liabilities	$ 539
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	(133)
Property and equipment, net	$ 52
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:PropertyPlantAndEquipmentNet
Finance lease liabilities	$ 90
Finance Lease, Liability, Statement of Financial Position [Extensible List]	amsf:AccountsPayableAndOtherLiabilities