Stock Options and Stock-Based Compensation - Stock Compensation Expense for Time Based Options and Performance Based Options (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Effect on basic earnings per share	$ 0.83	$ 0.64	$ 0.66	$ 0.46	$ 0.65	$ 0.51	$ 0.70	$ 0.72	$ 2.59	$ 2.59	$ 2.48
Effect on diluted earnings per share	$ 0.81	$ 0.63	$ 0.65	$ 0.46	$ 0.64	$ 0.50	$ 0.69	$ 0.71	$ 2.55	$ 2.55	$ 2.46
Time Based Options and Performance Based Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total cost of stock-based compensation included in income before income taxes									$ 4,978,000	$ 4,485,000	$ 4,349,000
Amount of income tax benefit recognized									1,057,000	985,000	1,048,000
Amount charged to net income									$ 3,921,000	$ 3,500,000	$ 3,301,000
Effect on basic earnings per share									$ 0.22	$ 0.19	$ 0.18
Effect on diluted earnings per share									$ 0.22	$ 0.19	$ 0.17
Time Based Options and Performance Based Options [Member] | Cost of Revenue [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total cost of stock-based compensation included in income before income taxes									$ 2,020,000	$ 2,028,000	$ 1,896,000
Time Based Options and Performance Based Options [Member] | General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total cost of stock-based compensation included in income before income taxes									$ 2,958,000	$ 2,457,000	$ 2,453,000