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Fair Value Measurements - Table 1 (Details) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Market Approach Valuation Technique [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Current portion of restricted marketable securities $ 10.2 $ 17.8
Noncurrent portion of restricted marketable securities 45.9 44.2
Market Approach Valuation Technique [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Current portion of restricted marketable securities 0.0 0.0
Noncurrent portion of restricted marketable securities 0.0 0.0
Market Approach Valuation Technique [Member] | Significant Other Observable Inputs (Level 2) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Current portion of restricted marketable securities 10.2 17.8
Noncurrent portion of restricted marketable securities 45.9 44.2
Market Approach Valuation Technique [Member] | Significant Unobservable Inputs (Level 3) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Current portion of restricted marketable securities 0.0 0.0
Noncurrent portion of restricted marketable securities 0.0 0.0
Income Approach Valuation Technique [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Redeemable noncontrolling interests 198.6 220.9
Income Approach Valuation Technique [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Redeemable noncontrolling interests 0.0 0.0
Income Approach Valuation Technique [Member] | Significant Other Observable Inputs (Level 2) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Redeemable noncontrolling interests 0.0 0.0
Income Approach Valuation Technique [Member] | Significant Unobservable Inputs (Level 3) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Redeemable noncontrolling interests $ 198.6 $ 220.9