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Investments - Investment Securities, Aggregated by Investment Type and Length of Time (Details)
$ in Millions
Dec. 27, 2020
USD ($)
security
Jun. 28, 2020
USD ($)
security
Debt Securities, Available-for-sale [Line Items]    
Fair value of securities in an unrealized loss position for less than 12 months $ 12.0 $ 65.8
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 12.0 65.8
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
Number of securities in an unrealized loss position for less than 12 months | security 10 46
Number of securities in an unrealized loss position for greater than 12 months | security 0 0
Number of securities in an unrealized loss position | security 10 46
Maximum    
Debt Securities, Available-for-sale [Line Items]    
Accumulated unrealized loss of securities in an unrealized loss position for less than 12 months $ (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 1.0 14.3
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 1.0 14.3
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
Corporate bonds    
Debt Securities, Available-for-sale [Line Items]    
Fair value of securities in an unrealized loss position for less than 12 months 7.0 29.1
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 7.0 29.1
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. agency securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value of securities in an unrealized loss position for less than 12 months 2.0 8.6
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 2.0 8.6
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 13.8
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 2.0 13.8
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