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, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Effect on basic earnings per share	$ 0.64	$ 0.55	$ 0.68	$ 0.62	$ 0.47	$ 0.51	$ 0.45	$ 0.47	$ 2.48	$ 1.90	$ 1.52
Effect on diluted earnings per share	$ 0.63	$ 0.54	$ 0.67	$ 0.62	$ 0.47	$ 0.50	$ 0.44	$ 0.46	$ 2.46	$ 1.87	$ 1.51
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									$ 4,349,000	$ 3,164,000	$ 2,432,000
Amount of income tax benefit recognized									1,048,000	1,105,000	946,000
Amount charged to net income									$ 3,301,000	$ 2,059,000	$ 1,486,000
Effect on basic earnings per share									$ 0.18	$ 0.11	$ 0.08
Effect on diluted earnings per share									$ 0.17	$ 0.11	$ 0.08
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									$ 1,896,000	$ 1,749,000	$ 1,507,000
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									$ 2,453,000	$ 1,415,000	$ 925,000