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Differences Between Statutory Income Tax Rate and Effective Income Tax Rate (Detail)
1 Months Ended 3 Months Ended 6 Months Ended
Dec. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Income Tax Disclosure [Abstract]          
Statutory income tax rate 35.00% 21.00% 35.00% 21.00% 35.00%
Excess tax benefits from stock-based awards   (24.90%) (13.70%) (66.60%) (32.10%)
Credits   (1.80%) (1.20%) (1.80%) (1.20%)
State taxes, net   4.30% 3.50% 4.30% 3.50%
Permanent differences   1.80% 1.80% 1.80% 1.80%
Other   2.90% (0.80%) 1.60% (0.40%)
Effective tax rate   3.30% 24.60% (39.70%) 6.60%