Fair Value Measurements - Table 1 (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Valuation, Market Approach [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Current portion of restricted marketable securities		$ 17.8
Noncurrent portion of restricted marketable securities	$ 61.4	44.2
Valuation, Market Approach [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
Noncurrent portion of restricted marketable securities	0.0	0.0
Valuation, Market Approach [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Current portion of restricted marketable securities		17.8
Noncurrent portion of restricted marketable securities	61.4	44.2
Valuation, Market Approach [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
Noncurrent portion of restricted marketable securities	0.0	0.0
Valuation, Income Approach [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Redeemable noncontrolling interests	236.5	220.9
Valuation, Income Approach [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
Valuation, Income Approach [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
Valuation, Income Approach [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Redeemable noncontrolling interests	$ 236.5	$ 220.9