Employee Stock Purchase Plan - Summary of Employee Stock Purchase Plan (Detail) - USD ($)	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Employee Stock Purchase Plan [Line Items]
Employee contributions	$ 458,000	$ 419,000	$ 371,000
Shares acquired	2,469,843
Employee Stock Purchase Plan [Member]
Employee Stock Purchase Plan [Line Items]
Shares acquired	8,976	10,596	10,975
Average purchase price	$ 49.78	$ 38.80	$ 33.81