Measured on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	$ 1,295.3	$ 501.6
Equity Securities
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	2.8	1.6
Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	542.9	207.8
Time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	11.5
Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	723.2	226.9
Municipal bonds- tax exempt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		55.3
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Commercial Paper	118.7
Total assets measured at fair value	2,196.6	1,410.3
Fair Value, Measurements, Recurring | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Government agency bonds	20.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Commercial Paper	0.0
Total assets measured at fair value	123.8	345.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Government agency bonds	0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Commercial Paper	118.7
Total assets measured at fair value	2,072.8	1,064.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Government agency bonds	20.0
Fair Value, Measurements, Recurring | Cash and Cash Equivalents | Money Market Funds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	121.0	343.8
Fair Value, Measurements, Recurring | Cash and Cash Equivalents | Demand and time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	641.6	564.9
Fair Value, Measurements, Recurring | Cash and Cash Equivalents | Fair Value, Inputs, Level 1 | Money Market Funds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	121.0	343.8
Fair Value, Measurements, Recurring | Cash and Cash Equivalents | Fair Value, Inputs, Level 1 | Demand and time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	0.0	0.0
Fair Value, Measurements, Recurring | Cash and Cash Equivalents | Fair Value, Inputs, Level 2 | Money Market Funds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	0.0	0.0
Fair Value, Measurements, Recurring | Cash and Cash Equivalents | Fair Value, Inputs, Level 2 | Demand and time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	641.6	564.9
Fair Value, Measurements, Recurring | Short-term Investments | Equity Securities
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	2.8	1.6
Fair Value, Measurements, Recurring | Short-term Investments | Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	542.9	165.2
Fair Value, Measurements, Recurring | Short-term Investments | Time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	11.5
Fair Value, Measurements, Recurring | Short-term Investments | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	723.2	162.0
Fair Value, Measurements, Recurring | Short-term Investments | Municipal bonds- tax exempt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		55.3
Fair Value, Measurements, Recurring | Short-term Investments | Municipal bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	14.9	10.0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 1 | Equity Securities
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	2.8	1.6
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 1 | Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	0.0	0.0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 1 | Time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	0.0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 1 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	0.0	0.0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 1 | Municipal bonds- tax exempt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		0.0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 1 | Municipal bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	0.0	0.0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 2 | Equity Securities
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	0.0	0.0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 2 | Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	542.9	165.2
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 2 | Time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	11.5
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 2 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	723.2	162.0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 2 | Municipal bonds- tax exempt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		55.3
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 2 | Municipal bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	$ 14.9	10.0
Fair Value, Measurements, Recurring | Other Long-term Investments | Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		42.6
Fair Value, Measurements, Recurring | Other Long-term Investments | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		64.9
Fair Value, Measurements, Recurring | Other Long-term Investments | Fair Value, Inputs, Level 1 | Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		0.0
Fair Value, Measurements, Recurring | Other Long-term Investments | Fair Value, Inputs, Level 1 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		0.0
Fair Value, Measurements, Recurring | Other Long-term Investments | Fair Value, Inputs, Level 2 | Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		42.6
Fair Value, Measurements, Recurring | Other Long-term Investments | Fair Value, Inputs, Level 2 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		$ 64.9