Employee Benefit Plan - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan name	401(k) benefit plan
Defined Contribution Plan, Plan Name [Extensible List]	amsf:KBenefitPlanMember
Matches employee compensation for participating employees	50.00%
Fully vested in employer contributions	5 years
Defined contribution this plan	$ 0.7	$ 0.7	$ 0.7
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Matches employee compensation for participating employees	6.00%