Summary of Unaudited Quarterly Results of Operations (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Mar. 31, 2020		Dec. 31, 2019	[1]	Sep. 30, 2019	[1]	Jun. 30, 2019	[1]	Mar. 31, 2019		Dec. 31, 2018	[1]	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2020		Mar. 31, 2019		Mar. 31, 2018
Schedule Of Quarterly Financial Data [Line Items]
Income tax provision (benefit)	$ 7,515	[1]	$ 27,799		$ 4,287		$ 14,215		$ 24,569	[1]	$ 19,648		$ 1,706		$ (41,579)		$ 53,816	[1]	$ 4,344	[1]	$ 48,267
Vested Restricted Stock Units or Stock Options
Schedule Of Quarterly Financial Data [Line Items]
Income tax provision (benefit)																	(14,800)		(69,300)
Shockwave Medical, Inc.
Schedule Of Quarterly Financial Data [Line Items]
Investments									$ 25,000										25,000
Fair value of investment	$ 55,700																55,700
Shockwave Medical, Inc. | Other Expense
Schedule Of Quarterly Financial Data [Line Items]
Pre-tax gain (loss) recorded in other income																	$ (500)
Shockwave Medical, Inc. | Other Income
Schedule Of Quarterly Financial Data [Line Items]
Pre-tax gain (loss) recorded in other income																			$ 32,000

[1]   (2) The income tax provision for the years ended March 31, 2020 and 2019 included excess tax benefits of $14.8 million and $69.3 million, respectively. These recognized excess tax benefits resulted from restricted stock units that vested or stock options that were exercised during the years ended March 31, 2020 and 2019.