Organization and Summary of Significant Accounting Policies - Schedule of Recast of Certain Prior Period Amounts to Conform with the Adoption of Topic 606 (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended												12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018		Mar. 31, 2018	[1]	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2019	Jun. 30, 2018		Jun. 30, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue	$ 215.4	$ 216.6	$ 216.8	$ 202.3	$ 193.9	[1]	$ 183.1		$ 187.9		$ 178.8		$ 851.1	$ 743.7	[1],[2]	$ 728.7	[1],[2]
Selling, general, and administrative expense	149.8	140.6	135.2	129.9	112.5	[1]	107.9		107.4		107.2		555.5	435.0	[1]	439.9	[1]
Income tax provision (benefit)	(3.4)	(3.6)	1.0	1.6	3.5	[1]	4.3		(25.3)		4.5		(4.4)	(13.0)	[1]	19.0	[1]
Net income attributable to Myriad Genetics, Inc. stockholders	$ (4.2)	$ 6.9	$ 2.6	$ (0.7)	$ 14.5	[1]	$ 9.1		$ 30.9		$ 78.8		$ 4.6	$ 133.3	[1]	$ 17.4	[1]
Earnings per share:
Basic	$ (0.06)	$ 0.09	$ 0.04	$ (0.01)	$ 0.21	[1]	$ 0.13		$ 0.45		$ 1.15		$ 0.06	$ 1.92	[1]	$ 0.25	[1]
Diluted	$ (0.06)	$ 0.09	$ 0.03	$ (0.01)	$ 0.20	[1]	$ 0.13		$ 0.43		$ 1.12		$ 0.06	$ 1.85	[1]	$ 0.25	[1]
Trade accounts receivable	$ 133.9				$ 99.5	[1]							$ 133.9	$ 99.5	[1]	$ 89.2
Retained earnings	$ 25.6				54.1	[1]							$ 25.6	54.1	[1]	(79.2)
As Reported | ASC 606
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue														772.6		769.9
Selling, general, and administrative expense														467.1		476.4
Income tax provision (benefit)														(14.0)		20.6
Net income attributable to Myriad Genetics, Inc. stockholders														$ 131.1		$ 20.5
Earnings per share:
Basic														$ 1.89		$ 0.30
Diluted														$ 1.82		$ 0.30
Trade accounts receivable					98.3									$ 98.3		$ 90.2
Retained earnings					52.9									52.9		(78.2)
Adjusted for ASC 606 | ASC 606
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue														(28.9)		(41.2)
Selling, general, and administrative expense														(32.1)		(36.5)
Income tax provision (benefit)														1.0		(1.6)
Net income attributable to Myriad Genetics, Inc. stockholders														$ 2.2		$ (3.1)
Earnings per share:
Basic														$ 0.03		$ (0.05)
Diluted														$ 0.03		$ (0.05)
Trade accounts receivable					1.2									$ 1.2		$ (1.0)
Retained earnings					$ 1.2									$ 1.2		$ (1.0)

[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 of this report for additional information.

[2] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606)