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2. Available-for-Sale Investments (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] $ 356.5  
Debt Securities, Available-for-sale [1] 359.9  
Fair Value, Recurring [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 356.5 $ 262.2
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 3.4 5.4
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] 0.0 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0 0.0
Debt Securities, Available-for-sale [1] 359.9 267.6
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] 245.2 133.2
Fair Value, Recurring [Member] | Municipal obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] 17.9 15.2
Fair Value, Recurring [Member] | US Government Sponsored Agencies [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] 58.5 76.9
Fair Value, Recurring [Member] | Foreign Government Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] 3.2 4.0
Fair Value, Recurring [Member] | Foreign Government Debt [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] 2.1 2.1
Fair Value, Recurring [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] 28.1 36.2
Fair Value, Recurring [Member] | Other Long-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 0.1 0.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 0.0 0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] 0.0 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0 0.0
Debt Securities, Available-for-sale [1] 0.1 0.1
Fair Value, Recurring [Member] | Other Long-term Investments [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 0.1 0.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 0.0 0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] 0.0 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0 0.0
Debt Securities, Available-for-sale [1] 0.1 0.1
Fair Value, Recurring [Member] | Short-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 356.4 262.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 3.4 5.4
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] 0.0 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0 0.0
Debt Securities, Available-for-sale [1] 359.8 267.5
Fair Value, Recurring [Member] | Short-term Investments [Member] | Corporate Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 243.3 130.5
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 1.9 2.7
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] 0.0 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0 0.0
Debt Securities, Available-for-sale [1] 245.2 133.2
Fair Value, Recurring [Member] | Short-term Investments [Member] | Municipal obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 17.8 15.0
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 0.1 0.2
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] 0.0 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0 0.0
Debt Securities, Available-for-sale [1] 17.9 15.2
Fair Value, Recurring [Member] | Short-term Investments [Member] | US Government Sponsored Agencies [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 57.3 74.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 1.2 2.2
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] 0.0 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0 0.0
Debt Securities, Available-for-sale [1] 58.5 76.9
Fair Value, Recurring [Member] | Short-term Investments [Member] | Foreign Government Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 3.2 4.0
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 0.0 0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] 0.0 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0 0.0
Debt Securities, Available-for-sale [1] 3.2 4.0
Fair Value, Recurring [Member] | Short-term Investments [Member] | Foreign Government Debt [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 2.1 2.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 0.0 0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] 0.0 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0 0.0
Debt Securities, Available-for-sale [1] 2.1 2.1
Fair Value, Recurring [Member] | Short-term Investments [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 27.8 35.8
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 0.2 0.3
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] 0.0 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0 0.0
Debt Securities, Available-for-sale [1] 28.0 36.1
Fair Value, Recurring [Member] | Other Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] $ 0.1 $ 0.1
[1] Available-for-sale investments are included in the following accounts in the condensed consolidated balance sheets (in millions):
 June 30, 2021December 31, 2020
Short-term investments$359.8 $267.5 
Other investments0.1 0.1 
Total$359.9 $267.6