Summary of Unaudited Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018		Mar. 31, 2017	Mar. 31, 2016
Disclosure Unaudited Quarterly Financial Information [Abstract]
Revenue	$ 174,436		$ 154,022		$ 132,823		$ 132,468		$ 124,680	$ 114,674	$ 102,955	$ 102,995	$ 593,749		$ 445,304	$ 329,543
Cost of revenue	30,098		24,994		21,627		21,862		19,261	18,987	17,309	15,070	98,581		70,627	50,419
Other operating expenses	96,771		84,262		79,470		77,528		76,425	70,284	71,138	66,692	338,031		284,539
Other income, net	940		888		758		714		362	423	228	192	3,300		1,205	734
Income before income taxes	48,507		45,654		32,484		33,792		29,356	25,826	14,736	21,425	160,437		91,343	65,838
Income tax (benefit) provision	11,660	[1],[2]	32,208	[1],[2]	7,981	[1],[2]	(3,582)	[1],[2]	14,457	10,394	5,861	8,515	48,267	[1],[2]	39,227	27,691
Net income	$ 36,847		$ 13,446		$ 24,503		$ 37,374		$ 14,899	$ 15,432	$ 8,875	$ 12,910	$ 112,170		$ 52,116	$ 38,147
Basic net income per share	$ 0.83		$ 0.30		$ 0.56		$ 0.85		$ 0.34	$ 0.36	$ 0.21	$ 0.30	$ 2.54		$ 1.21	$ 0.90
Diluted net income per share	$ 0.80		$ 0.29		$ 0.54		$ 0.82		$ 0.33	$ 0.34	$ 0.20	$ 0.29	$ 2.45		$ 1.17	$ 0.85

[1] In the first quarter of fiscal 2018, the Company adopted Accounting Standards Update No. 2016-09, Improvements to Employee Share-Based Payment Accounting, which requires that all excess tax benefits and tax deficiencies related share-based compensation arrangements be recognized as income tax benefit or expense, instead of in stockholders’ equity as previous guidance required. The income tax benefit for the year ended March 31, 2018 included excess tax benefits of $31.0 million. These recognized excess tax benefits resulted from restricted stock units that vested or stock options that were exercised during the year ended March 31, 2018.

[2] On December 22, 2017, the Tax Cuts and Jobs Act, or Tax Reform Act, was enacted into law. This new law, among other items, reduces the U.S. federal statutory corporate income tax rate from 35% to 21% effective January 1, 2018. During the year ended March 31, 2018, the Company recorded tax expense adjustments for $21.4 million related to the revaluation of its deferred taxes due to a reduction of the U.S. federal statutory corporate income tax rate.