Fair Value Measurements (Details) - USD ($)	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Total assets measured at fair value	$ 430,900,000	$ 2,196,600,000
Money market mutual funds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	8,300,000	121,000,000.0
Deposit accounts
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	420,300,000	641,600,000
Commercial paper
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents		118,700,000
Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents		20,000,000.0
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Total assets measured at fair value	10,600,000	123,800,000
Fair Value, Inputs, Level 1 | Money market mutual funds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	8,300,000	121,000,000.0
Fair Value, Inputs, Level 1 | Deposit accounts
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	0	0
Fair Value, Inputs, Level 1 | Commercial paper
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents		0
Fair Value, Inputs, Level 1 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents		0
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Total assets measured at fair value	420,300,000	2,072,800,000
Fair Value, Inputs, Level 2 | Money market mutual funds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	0	0
Fair Value, Inputs, Level 2 | Deposit accounts
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	420,300,000	641,600,000
Fair Value, Inputs, Level 2 | Commercial paper
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents		118,700,000
Fair Value, Inputs, Level 2 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents		20,000,000.0
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Impairment charges	0	0	$ 0
Short-term Investments | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Investments		723,200,000
Short-term Investments | Marketable equity securities
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Investments	2,300,000	2,800,000
Short-term Investments | Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Investments		542,900,000
Short-term Investments | Time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Investments		11,500,000
Short-term Investments | Municipal bonds - taxable
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Investments		14,900,000
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]
Investments		0
Short-term Investments | Fair Value, Inputs, Level 1 | Marketable equity securities
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Investments	2,300,000	2,800,000
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]
Investments		0
Short-term Investments | Fair Value, Inputs, Level 1 | Time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Investments		0
Short-term Investments | Fair Value, Inputs, Level 1 | Municipal bonds - taxable
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Investments		0
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]
Investments		723,200,000
Short-term Investments | Fair Value, Inputs, Level 2 | Marketable equity securities
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Investments	$ 0	0
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]
Investments		542,900,000
Short-term Investments | Fair Value, Inputs, Level 2 | Time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Investments		11,500,000
Short-term Investments | Fair Value, Inputs, Level 2 | Municipal bonds - taxable
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Investments		$ 14,900,000