Income Taxes - Additional Information (Detail) - USD ($)			1 Months Ended	3 Months Ended																12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2017	Mar. 31, 2019		Dec. 31, 2018	[1]	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018		Dec. 31, 2017	[1],[2]	Sep. 30, 2017	[1],[2]	Jun. 30, 2017	[1],[2]	Mar. 31, 2019		Mar. 31, 2018		Mar. 31, 2017
Income Taxes [Line Items]
U.S. federal statutory corporate tax rate	21.00%	35.00%	35.00%																	21.00%		31.50%		35.00%
Blended U.S. federal statutory corporate income tax rate																						31.50%
Measurement period																				1 year
Income tax expense																				$ 0		$ 21,400,000
Potential repatriation of foreign subsidiary earnings																				100.00%
Additional federal income tax expense																				$ 500,000
Income tax provision				$ 24,569,000	[1]	$ 19,648,000		$ 1,706,000		$ (41,579,000)		$ 11,660,000	[1],[2]	$ 32,208,000		$ 7,981,000		$ (3,582,000)		$ 4,344,000	[1]	$ 48,267,000	[1],[2]	$ 39,227,000
Effective income tax rate																				1.60%		30.10%		43.00%
Excess tax benefits from stock-based awards																								$ 12,038,000
Valuation allowance				1,302,000								$ 1,652,000								$ 1,302,000		$ 1,652,000
Net operating loss carry forwards				116,600,000																116,600,000
Foreign tax credits																				9,800,000
Interest and penalties on uncertain tax positions																				$ 0		0		$ 0
Earliest Tax Year
Income Taxes [Line Items]
Net operating loss carry forwards expiration period																				2029
Foreign tax credits carry forwards expiration period																				2022
Federal and state research and development credit carry forwards year of expiration																				2020
Latest Tax Year
Income Taxes [Line Items]
Net operating loss carry forwards expiration period																				2034
Foreign tax credits carry forwards expiration period																				2028
Federal and state research and development credit carry forwards year of expiration																				2039
Indefinite
Income Taxes [Line Items]
Net operating loss carry forwards				42,200,000																$ 42,200,000
Foreign
Income Taxes [Line Items]
Net operating loss carry forwards				2,800,000																2,800,000
Federal
Income Taxes [Line Items]
Research and development credit carry forwards				19,000,000																19,000,000
State
Income Taxes [Line Items]
Research and development credit carry forwards				$ 10,100,000																10,100,000
ASU 2016-09
Income Taxes [Line Items]
Excess tax benefits from stock-based awards																				$ 69,300,000		$ 31,000,000

[1] 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.

[2] 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.