Employee Stock Purchase Plan - Summary of Employee Stock Purchase Plan (Detail) - USD ($)	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Employee Stock Purchase Plan [Line Items]
Employee contributions	$ 503,000	$ 458,000	$ 419,000
Shares acquired	2,478,114
Employee Stock Purchase Plan [Member]
Employee Stock Purchase Plan [Line Items]
Shares acquired	8,271	8,976	10,596
Average purchase price	$ 59.55	$ 49.78	$ 38.80