Selected Quarterly Financial Data (Unaudited) - Unaudited Quarterly Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Statement of Operations Data:
Revenue	$ 251,671	$ 227,868	$ 153,616	$ 359,429	$ 294,978	$ 101,716	$ 17,265	$ 7,753	$ 992,584	$ 421,712	$ 32,528
Cost of revenue	62,134	43,466	35,858	74,075	51,772	26,261	7,717	4,057	215,533	89,807	14,107
Gross profit	189,537	184,402	117,758	285,354	243,206	75,455	9,548	3,696	777,051	331,905	18,421
Operating expenses:
Research and development	7,464	6,021	5,312	5,422	4,576	3,177	1,849	1,978	24,219	11,580	6,537
Selling and marketing	8,200	6,012	5,219	5,008	5,081	5,014	3,260	1,597	24,439	14,952	5,898
General and administrative	22,102	12,299	8,329	8,002	7,640	3,741	1,799	2,035	50,732	15,215	6,414
Amortization of intangible assets	911	797							1,708
Total operating expenses	38,677	25,129	18,860	18,432	17,297	11,932	6,908	5,610	101,098	41,747	18,849
Operating income (loss)	150,860	159,273	98,898	266,922	225,909	63,523	2,640	(1,914)	675,953	290,158	(428)
Interest and other (expense) income, net	(35)	496	604	282	589	421	275	241
Income before income taxes, equity loss in investee and gain (loss) on equity-method investments	150,825	159,769	99,502	267,204	226,498	63,944	2,915	(1,673)	677,300	291,684	409
Provision for (benefit from) income taxes	47,148	37,545	23,589	66,513	58,571	14,526	(599)	34	174,795	72,532	43
Income (loss) before equity earnings (loss) in investee and gain (loss) on equity-method investments	103,677	122,224	75,913	200,691	167,927	49,418	3,514	(1,707)
Equity earnings (loss) in investee					143	(189)	(193)	(249)		(488)	(777)
Gain (loss) on equity-method investments			3,734		(1,763)	(2,591)			3,734	(4,354)
Net income (loss) from consolidated operations	103,677	122,224	79,647	200,691	166,307	46,638	3,321	(1,956)	506,239	214,310	(411)
Net loss attributable to noncontrolling interests	662	298	165						1,125
Net income (loss) attributable to Fulgent	$ 104,339	$ 122,522	$ 79,812	$ 200,691	$ 166,307	$ 46,638	$ 3,321	$ (1,956)	$ 507,364	$ 214,310	$ (411)
Net income (loss) per common share attributable to Fulgent:
Basic	$ 3.48	$ 4.13	$ 2.74	$ 6.96	$ 6.55	$ 2.11	$ 0.15	$ (0.09)	$ 17.25	$ 9.44	$ (0.02)
Diluted	$ 3.34	$ 3.93	$ 2.59	$ 6.52	$ 6.16	$ 1.98	$ 0.14	$ (0.09)	$ 16.38	$ 8.91	$ (0.02)