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3. Fair Value Measurements Available-for-Sale Investments (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost $ 400.9  
Debt Securities, Available-for-sale 402.9  
Fair Value, Recurring [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale 402.9 $ 373.2
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale 204.5 215.0
Fair Value, Recurring [Member] | Municipal Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale 11.6 11.0
Fair Value, Recurring [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale 72.9 63.3
Fair Value, Recurring [Member] | US Government Sponsored Agencies [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale 106.1 80.3
Fair Value, Recurring [Member] | Foreign Government Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale 4.7 3.6
Fair Value, Recurring [Member] | Foreign Government Debt [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale 3.1  
Fair Value, Recurring [Member] | Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 400.9 375.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 2.4 0.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax (0.4) (2.6)
Debt Securities, Available-for-sale 402.9 373.2
Fair Value, Recurring [Member] | Short-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale 402.8 373.0
Fair Value, Recurring [Member] | Short-term Investments [Member] | Corporate Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 203.2 216.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 1.4 0.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax (0.1) (1.3)
Debt Securities, Available-for-sale 204.5 215.0
Fair Value, Recurring [Member] | Short-term Investments [Member] | Municipal Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 11.5 11.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 0.1 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 0.0 (0.1)
Debt Securities, Available-for-sale 11.6 11.0
Fair Value, Recurring [Member] | Short-term Investments [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 72.7 63.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 0.2 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax (0.1) (0.4)
Debt Securities, Available-for-sale 72.8 63.1
Fair Value, Recurring [Member] | Short-term Investments [Member] | US Government Sponsored Agencies [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 105.6 80.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 0.7 0.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax (0.2) (0.8)
Debt Securities, Available-for-sale 106.1 80.3
Fair Value, Recurring [Member] | Short-term Investments [Member] | Foreign Government Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 4.7 3.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 0.0 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 0.0 0.0
Debt Securities, Available-for-sale 4.7 3.6
Fair Value, Recurring [Member] | Short-term Investments [Member] | Foreign Government Debt [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 3.1  
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 0.0  
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 0.0  
Debt Securities, Available-for-sale 3.1  
Fair Value, Recurring [Member] | Short-term Investments [Member] | Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 400.8 375.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 2.4 0.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax (0.4) (2.6)
Debt Securities, Available-for-sale 402.8 373.0
Fair Value, Recurring [Member] | Other Long-term Investments [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 0.1 0.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 0.0 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 0.0 0.0
Debt Securities, Available-for-sale 0.1 0.2
Fair Value, Recurring [Member] | Other Long-term Investments [Member] | Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 0.1 0.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 0.0 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 0.0 0.0
Debt Securities, Available-for-sale $ 0.1 $ 0.2