Segment and Related Information - Schedule of Total Operating Income for Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting [Abstract]
Total operating income (loss) for reportable segments	$ (133.7)			$ 5.9			$ (163.4)	$ 13.2
Interest income	0.8			0.7			2.5	2.3
Interest expense	(2.3)			(3.2)			(7.7)	(8.8)
Other	4.1			(0.1)			3.8	1.0
Income (loss) before income tax	(131.1)			3.3			(164.8)	7.7
Income tax provision (benefit)	(15.9)			(3.6)			(20.7)	(1.0)
Net income (loss)	(115.2)			6.9			(144.1)	8.7
Net loss attributable to non-controlling interest								(0.1)
Net income (loss) attributable to Myriad Genetics, Inc. stockholders	$ (115.2)	$ (8.3)	$ (20.6)	$ 6.9	$ 2.6	$ (0.7)	$ (144.1)	$ 8.8