Financial Instruments Measured at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	$ 657,761	$ 593,589
Contingent consideration	24,417	9,000
Cross Currency Interest Rate Contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative fair value	2,860	3,786
Repurchase Agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	20,000	20,000
Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	330,843	250,775
Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	202,613	207,795
Shockwave Medical
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	22,494	55,704
Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	104,305	115,019
Commercial Paper | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	40,017	32,831
Level 1 | Shockwave Medical
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	22,494	55,704
Level 1 | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	104,305	115,019
Level 2 | Cross Currency Interest Rate Contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative fair value	2,860	3,786
Level 2 | Repurchase Agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	20,000	20,000
Level 2 | Commercial Paper | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	40,017	32,831
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration	24,417	9,000
U.S. Treasury mutual fund securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	75,589	42,648
U.S. Treasury mutual fund securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	10,248
U.S. Treasury mutual fund securities | Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	75,589	42,648
Government-backed securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	85,456	67,995
Government-backed securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	141,536	91,064
Government-backed securities | Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	85,456	67,995
Government-backed securities | Level 2 | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	141,536	91,064
Corporate Debt Securities | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	129,781	107,301
Corporate Debt Securities | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	50,829	116,731
Corporate Debt Securities | Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	129,781	107,301
Corporate Debt Securities | Level 2 | Long-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	50,829	$ 116,731
Long Term U.S. Treasury Mutual Fund Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	10,248
Long Term U.S. Treasury Mutual Fund Securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value disclosure	$ 10,248