Fair Value Measurements (Details) - USD ($) $ in Millions	Dec. 31, 2018	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value		$ 1,292.5
Total assets measured at fair value	$ 436.2	2,196.6
Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents:	6.7	121.0
Deposit accounts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents:	425.5	641.6
Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents:		118.7
Government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value		723.2
Government agency bonds		20.0
Marketable equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value	1.7	2.8
Corporate bonds and debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value		542.9
Time deposits
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value	2.3	11.5
Municipal bonds - taxable
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value		14.9
Quoted Prices in Active Markets for Identical Instruments (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	8.4	123.8
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents:	6.7	121.0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Deposit accounts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents:	0.0	0.0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents:		0.0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value		0.0
Government agency bonds		0.0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Marketable equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value	1.7	2.8
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Corporate bonds and debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value		0.0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Time deposits
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value	0.0	0.0
Quoted Prices in Active Markets for Identical Instruments (Level 1) | Municipal bonds - taxable
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value		0.0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	427.8	2,072.8
Significant Other Observable Inputs (Level 2) | Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents:	0.0	0.0
Significant Other Observable Inputs (Level 2) | Deposit accounts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents:	425.5	641.6
Significant Other Observable Inputs (Level 2) | Commercial Paper
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents:		118.7
Significant Other Observable Inputs (Level 2) | Government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value		723.2
Government agency bonds		20.0
Significant Other Observable Inputs (Level 2) | Marketable equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value	0.0	0.0
Significant Other Observable Inputs (Level 2) | Corporate bonds and debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value		542.9
Significant Other Observable Inputs (Level 2) | Time deposits
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value	$ 2.3	11.5
Significant Other Observable Inputs (Level 2) | Municipal bonds - taxable
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Estimated Fair Value		$ 14.9