Cash Equivalents and Marketable Securities (Parenthetical) (Detail)	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Maximum
Investment [Line Items]
Cash equivalents, maturity period	90 days	90 days
Short-term marketable securities, maturity period	1 year	1 year
Long-term marketable securities, maturity period	2 years	2 years
Minimum
Investment [Line Items]
Long-term marketable securities, maturity period	1 year	1 year