REVENUE RECOGNITION - Financial Statement Impact of Adoption (Details) - USD ($) $ / shares in Units, $ in Thousands	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	Apr. 01, 2018
ASSETS:
Accounts receivable and unbilled costs	$ 235,318				$ 213,438				$ 235,318	$ 213,438		$ 214,633
Inventories and deferred costs	26,270				34,774				26,270	34,774
Prepaid expenses and other current assets	35,658				33,502				35,658	33,502		38,128
Other assets	16,988				12,866				16,988	12,866		17,614
LIABILITIES:
Deferred revenue and customer deposits	272,508				301,925				272,508	301,925		271,698
Deferred tax liability	124,229				151,563				124,229	151,563		158,687
Long-term deferred revenue and customer deposits	94,619				91,409				94,619	91,409		90,157
STOCKHOLDERS' EQUITY:
Retained earnings	358,093				396,493				358,093	396,493		431,417
Income Statement [Abstract]
Total revenue	235,002	$ 246,008	$ 223,797	$ 205,111	235,224	$ 268,944	$ 256,863	$ 225,756	909,918	986,787	$ 1,162,112
Total cost of revenue									254,127	271,905	346,139
Sales and marketing expense									291,870	312,536	328,628
Income tax benefit									(19,588)	(98,471)	18,894
Net income (loss)	$ 19,211	$ (3,603)	$ (26,428)	$ (62,504)	$ 16,817	$ 89,685	$ (2,468)	$ (24,222)	$ (73,324)	$ 79,812	$ 33,291
Basic net loss per share (in dollars per share)									$ (0.93)	$ 0.91	$ 0.36
Diluted net loss per share (in dollars per share)	$ 0.24	$ (0.05)	$ (0.34)	$ (0.78)	$ 0.20	$ 1.02	$ (0.03)	$ (0.27)	$ (0.93)	$ 0.90	$ 0.36
Balance without Adoption of Topic 606
ASSETS:
Accounts receivable and unbilled costs	$ 229,708								$ 229,708
Inventories and deferred costs	26,418								26,418
Prepaid expenses and other current assets	31,810								31,810
Other assets	12,074								12,074
LIABILITIES:
Deferred revenue and customer deposits	306,799								306,799
Deferred tax liability	121,535								121,535
Long-term deferred revenue and customer deposits	106,025								106,025
STOCKHOLDERS' EQUITY:
Retained earnings	339,765								339,765
Income Statement [Abstract]
Total revenue									888,950
Total cost of revenue									253,979
Sales and marketing expense									292,080
Income tax benefit									(24,022)
Net income (loss)									$ (89,920)
Basic net loss per share (in dollars per share)									$ (1.14)
Diluted net loss per share (in dollars per share)									$ (1.14)
ASU 2014-09 | Effect of Change Higher (Lower)
ASSETS:
Accounts receivable and unbilled costs	5,610								$ 5,610			1,195
Inventories and deferred costs	(148)								(148)
Prepaid expenses and other current assets	3,848								3,848			4,626
Other assets	4,914								4,914			4,748
LIABILITIES:
Deferred revenue and customer deposits	(34,291)								(34,291)			(30,227)
Deferred tax liability	2,694								2,694			7,124
Long-term deferred revenue and customer deposits	(11,406)								(11,406)			(1,252)
STOCKHOLDERS' EQUITY:
Retained earnings	$ 18,328								18,328			$ 34,924
Income Statement [Abstract]
Total revenue									20,968
Total cost of revenue									148
Sales and marketing expense									(210)
Income tax benefit									4,434
Net income (loss)									$ 16,596
Basic net loss per share (in dollars per share)									$ 0.21
Diluted net loss per share (in dollars per share)									$ 0.21