Segment and Related Information - Schedule of Total Operating Income for Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	[1]	Dec. 31, 2018	Dec. 31, 2017	[1]
Segment Reporting [Abstract]
Total operating income for reportable segments	$ 6.1		$ 6.3		$ 7.3	$ 90.3
Interest income	0.9		0.4		1.6	0.8
Interest expense	(3.4)		(0.7)		(5.6)	(1.6)
Other	0.0		(0.4)		1.1	(0.7)
Income before income tax	3.6		5.6		4.4	88.8
Income tax provision	1.0		(25.3)		2.6	(20.8)
Net income	2.6		30.9		1.8	109.6
Net loss attributable to non-controlling interest					(0.1)	(0.1)
Net income attributable to Myriad Genetics, Inc. stockholders	$ 2.6	$ (0.7)	$ 30.9		$ 1.9	$ 109.7

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.