Stock-Based Compensation and Stock Options - Outstanding Options (Detail) - USD ($)	3 Months Ended	6 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options outstanding, beginning balance, Shares	1,017,494	1,000,830	1,029,103	1,058,411
Granted, Number of Options	41,650	50,450	86,825	97,475
Exercised, Number of Options	(80,455)	(55,993)	(124,818)	(156,948)
Cancelled - forfeited, Number of Options	(263)
Cancelled - expired, Number of Options	(2,509)
Options outstanding, ending balance, Shares	975,917	990,064	975,917	990,064
Ending vested and expected to vest, Number of Options	838,745		838,745
Ending exercisable, Number of Options	426,263		426,263
Options outstanding, beginning balance, Weighted Average Exercise Price	$ 55.54	$ 47.05	$ 54.87	$ 45.17
Granted, Weighted Average Exercise Price per Share	79.49	87.49	65.55	79.17
Exercised, Weighted Average Exercise Price per Share	38.66	33.72	37.77	36.48
Cancelled - forfeited, Weighted Average Exercise Price per Share	64.07
Cancelled - expired, Weighted Average Exercise Price per Share	47.92
Options outstanding, ending balance, Weighted Average Exercise Price	57.97	$ 49.84	57.97	$ 49.84
Ending vested and expected to vest, Weighted Average Exercise Price per Share	47.03		47.03
Ending exercisable, Weighted Average Exercise Price per Share	$ 45.88		$ 45.88
Option outstanding, Weighted Average Remaining Contractual Life (Years)	3 years 3 months 18 days
Ending vested and expected to vest, Weighted Average Remaining Contractual Life (Years)	2 years 11 months 19 days
Ending exercisable, Weighted Average Remaining Contractual Life (Years)	2 years 6 months 29 days
Option outstanding, Aggregate Intrinsic Value	$ 26,974,518		$ 26,974,518
Ending vested and expected to vest, Aggregate Intrinsic Value	24,141,622		24,141,622
Ending exercisable, Aggregate Intrinsic Value	$ 16,882,794		$ 16,882,794

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of share options (or share units) exercised during the current period.

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