Computation of Basic and Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	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
Basic Net Income Per Share
Net income	$ 56,856	$ 61,868	$ 62,213	$ 44,588	$ 31,798	$ 69,217	$ 13,071	$ 88,923	$ 225,525	$ 203,009	$ 259,016
Weighted average shares - basic									45,140	45,179	44,911
Net income per share - basic	$ 1.26	$ 1.37	$ 1.38	$ 0.99	$ 0.71	$ 1.53	$ 0.29	$ 1.97	$ 5.00	$ 4.49	$ 5.77
Diluted Net Income Per Share
Net income	$ 56,856	$ 61,868	$ 62,213	$ 44,588	$ 31,798	$ 69,217	$ 13,071	$ 88,923	$ 225,525	$ 203,009	$ 259,016
Weighted average shares - basic									45,140	45,179	44,911
Effect of dilutive securities									534	637	1,240
Weighted average shares - diluted									45,674	45,816	46,151
Net income per share - diluted	$ 1.24	$ 1.35	$ 1.36	$ 0.98	$ 0.70	$ 1.51	$ 0.28	$ 1.93	$ 4.94	$ 4.43	$ 5.61