Differences Between Statutory Income Tax Rate and Effective Income Tax Rate (Detail)	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Statutory income tax rate	35.00%	35.00%	35.00%	35.00%
Excess tax benefits from stock-based awards	(13.70%)		(32.10%)
Credits	(1.20%)	(1.30%)	(1.20%)	(1.30%)
State taxes, net	3.50%	3.40%	3.50%	3.30%
Permanent differences	1.80%	2.70%	1.80%	2.80%
Other	(0.80%)		(0.40%)
Effective tax rate	24.60%	39.80%	6.60%	39.80%