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Fair Value Measurements (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments:   $ 1,292.5
Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Commercial Paper   118.7
Total assets measured at fair value $ 464.2 2,196.6
Corporate bonds and debt    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments:   542.9
Time deposits    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments: 2.2 11.5
Government agency bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments:   723.2
Government agency bonds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Government agency bonds   20.0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Commercial Paper   0.0
Total assets measured at fair value 15.1 123.8
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Government agency bonds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Government agency bonds   0.0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Commercial Paper   118.7
Total assets measured at fair value 449.1 2,072.8
Significant Other Observable Inputs (Level 2) | Government agency bonds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Government agency bonds   20.0
Cash and cash equivalents: | Money market mutual funds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash and cash equivalents: 12.8 121.0
Cash and cash equivalents: | Deposit accounts | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash and cash equivalents: 446.9 641.6
Cash and cash equivalents: | Quoted Prices in Active Markets for Identical Instruments (Level 1) | Money market mutual funds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash and cash equivalents: 12.8 121.0
Cash and cash equivalents: | Quoted Prices in Active Markets for Identical Instruments (Level 1) | Deposit accounts | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash and cash equivalents: 0.0 0.0
Cash and cash equivalents: | Significant Other Observable Inputs (Level 2) | Money market mutual funds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash and cash equivalents: 0.0 0.0
Cash and cash equivalents: | Significant Other Observable Inputs (Level 2) | Deposit accounts | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Cash and cash equivalents: 446.9 641.6
Short-term investments: | Marketable equity securities | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments: 2.3 2.8
Short-term investments: | Corporate bonds and debt | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments:   542.9
Short-term investments: | Time deposits | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments: 2.2 11.5
Short-term investments: | Government agency bonds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments:   723.2
Short-term investments: | Municipal bonds - taxable | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments:   14.9
Short-term investments: | Quoted Prices in Active Markets for Identical Instruments (Level 1) | Marketable equity securities | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments: 2.3 2.8
Short-term investments: | Quoted Prices in Active Markets for Identical Instruments (Level 1) | Corporate bonds and debt | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments:   0.0
Short-term investments: | Quoted Prices in Active Markets for Identical Instruments (Level 1) | Time deposits | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments: 0.0 0.0
Short-term investments: | Quoted Prices in Active Markets for Identical Instruments (Level 1) | Government agency bonds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments:   0.0
Short-term investments: | Quoted Prices in Active Markets for Identical Instruments (Level 1) | Municipal bonds - taxable | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments:   0.0
Short-term investments: | Significant Other Observable Inputs (Level 2) | Marketable equity securities | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments: 0.0 0.0
Short-term investments: | Significant Other Observable Inputs (Level 2) | Corporate bonds and debt | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments:   542.9
Short-term investments: | Significant Other Observable Inputs (Level 2) | Time deposits | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments: $ 2.2 11.5
Short-term investments: | Significant Other Observable Inputs (Level 2) | Government agency bonds | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments:   723.2
Short-term investments: | Significant Other Observable Inputs (Level 2) | Municipal bonds - taxable | Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Short-term investments:   $ 14.9