Income per Share - Reconciliation of Basic and Diluted Income Per Share Computations (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net income attributable to Fulgent	$ 122,522	$ 46,638	$ 403,025	$ 48,003
Weighted-average common shares—outstanding, basic	29,673	22,062	29,221	21,793
Weighted-average common shares—outstanding, diluted	31,170	23,539	30,906	23,135
Net income per common share, basic	$ 4.13	$ 2.11	$ 13.79	$ 2.20
Net income per common share, diluted	$ 3.93	$ 1.98	$ 13.04	$ 2.07