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2. Available-for-Sale Investments (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] $ 406.4  
Debt Securities, Available-for-sale [1] 409.1  
Fair Value, Recurring [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 406.4 $ 262.2
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 2.8 5.4
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] (0.1) 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0 0.0
Debt Securities, Available-for-sale [1] 409.1 267.6
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] 231.5 133.2
Fair Value, Recurring [Member] | Municipal obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] 19.4 15.2
Fair Value, Recurring [Member] | US Government Sponsored Agencies [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] 66.1 76.9
Fair Value, Recurring [Member] | Foreign Government Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] 1.9 4.0
Fair Value, Recurring [Member] | Foreign Government Debt [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] 5.6 2.1
Fair Value, Recurring [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] 68.9 36.2
Fair Value, Recurring [Member] | Other Long-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 0.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0
Debt Securities, Available-for-sale [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
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1]   0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1]   0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1]   0.0
Debt Securities, Available-for-sale [1]   0.1
Fair Value, Recurring [Member] | Short-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 406.4 262.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 2.8 5.4
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] (0.1) 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0 0.0
Debt Securities, Available-for-sale [1] 409.1 267.5
Fair Value, Recurring [Member] | Short-term Investments [Member] | Corporate Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 230.0 130.5
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 1.6 2.7
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax [1] (0.1) 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss [1] 0.0 0.0
Debt Securities, Available-for-sale [1] 231.5 133.2
Fair Value, Recurring [Member] | Short-term Investments [Member] | Municipal obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 19.3 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] 19.4 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] 65.1 74.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 1.0 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] 66.1 76.9
Fair Value, Recurring [Member] | Short-term Investments [Member] | Foreign Government Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 1.9 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] 1.9 4.0
Fair Value, Recurring [Member] | Short-term Investments [Member] | Foreign Government Debt [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 5.6 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] 5.6 2.1
Fair Value, Recurring [Member] | Short-term Investments [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost [1] 68.8 35.8
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax [1] 0.1 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] 68.9 36.1
Fair Value, Recurring [Member] | Other Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale [1] $ 0.0 $ 0.1
[1] Available-for-sale investments are included in the following accounts in the condensed consolidated balance sheets (in millions):
 September 30, 2021December 31, 2020
Short-term investments$409.1 $267.5 
Other investments— 0.1 
Total$409.1 $267.6