Summary of Unaudited Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2019		Mar. 31, 2018		Mar. 31, 2017
Disclosure Unaudited Quarterly Financial Information [Abstract]
Revenue	$ 207,081		$ 200,563		$ 181,778		$ 180,010		$ 174,436		$ 154,022		$ 132,823		$ 132,468		$ 769,432		$ 593,749		$ 445,304
Cost of revenue	34,848		34,023		29,846		30,850		30,098		24,994		21,627		21,862		129,567		98,581		70,627
Other operating expenses	106,844		104,185		101,612		102,412		96,771		84,262		79,470		77,528		415,053		338,031
Other income, net	33,141	[1]	2,155	[1]	1,513	[1]	1,739	[1]	940		888		758		714		38,548	[1]	3,300		1,205
Income before income taxes	98,530		64,510		51,833		48,487		48,507		45,654		32,484		33,792		263,360		160,437		91,343
Income tax provision (benefit)	24,569	[2]	19,648	[2]	1,706	[2]	(41,579)	[2]	11,660	[2],[3]	32,208	[2],[3]	7,981	[2],[3]	(3,582)	[2],[3]	4,344	[2]	48,267	[2],[3]	39,227
Net income	$ 73,961		$ 44,862		$ 50,127		$ 90,066		$ 36,847		$ 13,446		$ 24,503		$ 37,374		$ 259,016		$ 112,170		$ 52,116
Basic net income per share	$ 1.64		$ 1.00		$ 1.11		$ 2.02		$ 0.83		$ 0.30		$ 0.56		$ 0.85		$ 5.77		$ 2.54		$ 1.21
Diluted net income per share	$ 1.60		$ 0.97		$ 1.09		$ 1.95		$ 0.80		$ 0.29		$ 0.54		$ 0.82		$ 5.61		$ 2.45		$ 1.17

[1] In fiscal 2019, the Company invested $25.0 million in medical device company Shockwave Medical. The fair value of this investment as of March 31, 2019 was $56.2 million and the Company recognized a gain of $31.2 million in Other income.

[2] On December 22, 2017, the Tax Cuts and Jobs Act, or Tax Reform Act, was enacted into law. This new law, among other items, reduces the U.S. federal statutory corporate income tax rate from 35% to 21% effective January 1, 2018. During the year ended March 31, 2018, the Company recorded tax expense adjustments for $21.4 million related to the revaluation of its deferred taxes due to a reduction of the U.S. federal statutory corporate income tax rate.

[3] In the first quarter of fiscal 2018, the Company adopted ASU 2016-09, which requires that all excess tax benefits and tax deficiencies related share-based compensation arrangements be recognized as income tax benefit or expense, instead of in stockholders’ equity as previous guidance required. The income tax provision for the years ended March 31, 2019 and 2018 included excess tax benefits of $69.3 million and $31.0 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, 2019 and 2018.