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2. Available-for-Sale Investments (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 382.6  
Estimated Fair Value 379.5  
Fair Value, Measurements, Recurring [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities [1]   $ 1,301.4
Amortized Cost 382.6  
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0.1  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax (3.2)  
Estimated Fair Value [1] 379.5  
Available-for-sale Securities, Amortized Cost Basis   418.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax   884.8
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax   1.5
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities [1]   185.7
Estimated Fair Value [1] 223.5  
Fair Value, Measurements, Recurring [Member] | Brokered certificates of deposit [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities [1]   0.7
Fair Value, Measurements, Recurring [Member] | Municipal obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities [1]   15.0
Estimated Fair Value [1] 15.6  
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities [1]   55.6
Estimated Fair Value [1] 61.0  
Fair Value, Measurements, Recurring [Member] | Marketable Equity Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities [1]   973.4
Fair Value, Measurements, Recurring [Member] | US Government Sponsored Agencies [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities [1]   67.6
Estimated Fair Value [1] 76.1  
Fair Value, Measurements, Recurring [Member] | Foreign Government Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities [1]   3.4
Estimated Fair Value [1] 3.3  
Fair Value, Measurements, Recurring [Member] | Other Long-term Investments [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   0.2
Amortized Cost 0.2  
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0.0  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 0.0  
Estimated Fair Value 0.2  
Available-for-sale Securities, Amortized Cost Basis   0.2
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax   0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax   0.0
Fair Value, Measurements, Recurring [Member] | Other Long-term Investments [Member] | Marketable Equity Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   930.0
Available-for-sale Securities, Amortized Cost Basis   54.5
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax   875.5
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax   0.0
Fair Value, Measurements, Recurring [Member] | Other Long-term Investments [Member] | Available-for-sale Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   930.2
Available-for-sale Securities, Amortized Cost Basis   54.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax   875.5
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax   0.0
Fair Value, Measurements, Recurring [Member] | Other Long-term Investments [Member] | Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 0.2  
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0.0  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 0.0  
Estimated Fair Value 0.2  
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   371.2
Estimated Fair Value 379.3  
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Corporate Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   185.7
Amortized Cost 224.7  
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0.1  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax (1.3)  
Estimated Fair Value 223.5  
Available-for-sale Securities, Amortized Cost Basis   185.9
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax   0.3
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax   0.5
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Brokered certificates of deposit [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   0.7
Available-for-sale Securities, Amortized Cost Basis   0.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax   0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax   0.0
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Municipal obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   15.0
Amortized Cost 15.7  
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0.0  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax (0.1)  
Estimated Fair Value 15.6  
Available-for-sale Securities, Amortized Cost Basis   15.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax   0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax   0.1
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   55.4
Amortized Cost 61.1  
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0.0  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax (0.3)  
Estimated Fair Value 60.8  
Available-for-sale Securities, Amortized Cost Basis   55.6
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax   0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax   0.2
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Marketable Equity Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   43.4
Available-for-sale Securities, Amortized Cost Basis   34.4
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax   9.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax   0.0
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Available-for-sale Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   371.2
Available-for-sale Securities, Amortized Cost Basis   363.4
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax   9.3
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax   1.5
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | US Government Sponsored Agencies [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   67.6
Amortized Cost 77.6  
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0.0  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax (1.5)  
Estimated Fair Value 76.1  
Available-for-sale Securities, Amortized Cost Basis   68.3
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax   0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax   0.7
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Foreign Government Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   3.4
Amortized Cost 3.3  
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0.0  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 0.0  
Estimated Fair Value 3.3  
Available-for-sale Securities, Amortized Cost Basis   3.4
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax   0.0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax   0.0
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 382.4  
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0.1  
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax (3.2)  
Estimated Fair Value 379.3  
Fair Value, Measurements, Recurring [Member] | Other Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities   $ 930.2
Estimated Fair Value $ 0.2  
[1] Available-for-sale investments are included in the following accounts in the Condensed Consolidated Balance Sheets (in millions):
 
September 30, 2018
 
December 31, 2017
Short-term investments
$
379.3

 
$
371.2

Other investments
0.2

 
930.2

Total
$
379.5

 
$
1,301.4