Investments - Investment Securities, Aggregated by Investment Type and Length of Time (Details) $ in Millions	12 Months Ended
	Jun. 30, 2019 USD ($) security	Jun. 24, 2018 USD ($) security	Jun. 25, 2017 USD ($)
Debt Securities, Available-for-sale [Line Items]
Fair value of securities in an unrealized loss position for less than 12 months	$ 55.8	$ 162.9
Fair value of securities in an unrealized loss position for greater than 12 months	48.4	5.1
Fair value of securities in an unrealized loss position	104.2	168.0
Accumulated unrealized loss of securities in an unrealized loss position for less than 12 months	0.0	(2.0)
Accumulated unrealized loss of securities in an unrealized loss position for greater than 12 months	(0.1)	0.0
Accumulated unrealized loss of securities in an unrealized loss position	$ (0.1)	$ (2.0)
Number of securities in an unrealized loss position for less than 12 months | security	46	151
Number of securities in an unrealized loss position for greater than 12 months | security	47	6
Number of securities in an unrealized loss position | security	93	157
Loss (gain) on sale of investments, net	$ (0.1)	$ (0.1)	$ 0.1
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Fair value of securities in an unrealized loss position for less than 12 months	4.3	97.5
Fair value of securities in an unrealized loss position for greater than 12 months	29.8	3.6
Fair value of securities in an unrealized loss position	34.1	101.1
Accumulated unrealized loss of securities in an unrealized loss position for less than 12 months	0.0	(0.9)
Accumulated unrealized loss of securities in an unrealized loss position for greater than 12 months	(0.1)	0.0
Accumulated unrealized loss of securities in an unrealized loss position	(0.1)	(0.9)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Fair value of securities in an unrealized loss position for less than 12 months	41.8	61.5
Fair value of securities in an unrealized loss position for greater than 12 months	14.7	1.5
Fair value of securities in an unrealized loss position	56.5	63.0
Accumulated unrealized loss of securities in an unrealized loss position for less than 12 months	0.0	(1.1)
Accumulated unrealized loss of securities in an unrealized loss position for greater than 12 months	0.0	0.0
Accumulated unrealized loss of securities in an unrealized loss position	0.0	(1.1)
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Fair value of securities in an unrealized loss position for less than 12 months	7.7	3.9
Fair value of securities in an unrealized loss position for greater than 12 months	0.0	0.0
Fair value of securities in an unrealized loss position	7.7	3.9
Accumulated unrealized loss of securities in an unrealized loss position for less than 12 months	0.0	0.0
Accumulated unrealized loss of securities in an unrealized loss position for greater than 12 months	0.0	0.0
Accumulated unrealized loss of securities in an unrealized loss position	0.0	$ 0.0
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Fair value of securities in an unrealized loss position for less than 12 months	2.0
Fair value of securities in an unrealized loss position for greater than 12 months	3.9
Fair value of securities in an unrealized loss position	5.9
Accumulated unrealized loss of securities in an unrealized loss position for less than 12 months	0.0
Accumulated unrealized loss of securities in an unrealized loss position for greater than 12 months	0.0
Accumulated unrealized loss of securities in an unrealized loss position	$ 0.0