Summary of Unaudited Quarterly Results of Operations (Parenthetical) (Detail) - USD ($)			1 Months Ended	12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Schedule Of Quarterly Financial Data [Line Items]
Statutory income tax rate	21.00%	35.00%	35.00%	21.00%	31.50%	35.00%
Income tax expense				$ 0	$ 21,400,000
ASU 2016-09
Schedule Of Quarterly Financial Data [Line Items]
Excess tax benefits from stock-based awards				69,300,000	$ 31,000,000
Shockwave Medical, Inc.
Schedule Of Quarterly Financial Data [Line Items]
Investments				25,000,000
Fair value of investment				56,200,000
Shockwave Medical, Inc. | Other Income
Schedule Of Quarterly Financial Data [Line Items]
Gain recorded in other income				$ 31,200,000