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Investments - Investment Securities, Aggregated by Investment Type and Length of Time (Details)
$ in Millions
Sep. 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 $ 51.6 $ 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 51.6 65.8
Accumulated unrealized loss of securities in an unrealized loss position for less than 12 months (0.1) 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 $ (0.1) $ 0.0
Number of securities in an unrealized loss position for less than 12 months | security 16 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 16 46
Municipal bonds    
Debt Securities, Available-for-sale [Line Items]    
Fair value of securities in an unrealized loss position for less than 12 months $ 4.0 $ 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 4.0 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 7.8 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 7.8 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 16.0 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 16.0 8.6
Accumulated unrealized loss of securities in an unrealized loss position for less than 12 months (0.1) 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 (0.1) 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 23.8 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 23.8 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