SIGNIFICANT ACCOUNTING POLICIES - Revenue Standard Adopted (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands			3 Months Ended										12 Months Ended
		Jan. 01, 2016	Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Accounting Policies [Abstract]
Amortization period for deferred contract costs													5 years
Assets
Accounts receivable, unbilled receivables, and contract assets								$ 383,733							$ 383,733
Long-term unbilled receivables	[1]		$ 151,237					160,708					$ 151,237		160,708
Deferred income taxes								14,240							14,240
Deferred contract costs								37,924							37,924
Other assets								416,148							416,148
Total assets	[1]		982,553					1,012,753					982,553		1,012,753
Liabilities and Stockholders’ Equity
Deferred revenue	[1]		185,145					165,850					185,145		165,850
Long-term deferred revenue								3,706							3,706
Deferred income tax liabilities	[1]		6,939					38,463					6,939		38,463
Other liabilities								148,864							148,864
Total liabilities	[1]		361,022					356,883					361,022		356,883
Foreign currency translation adjustments	[1]		(13,073)					(6,473)					(13,073)		(6,473)
Retained earnings	[1]		510,863					509,697					510,863		509,697
Other equity								152,646							152,646
Total stockholders’ equity	[2]		621,531	[1]				655,870	[1]				621,531	[1]	655,870	[1]	$ 548,940		$ 526,987
Total liabilities and stockholders’ equity	[1]		982,553					1,012,753					982,553		1,012,753
Revenue
Total revenue			256,357		$ 203,263	$ 196,779	$ 235,182	254,605		$ 190,957	$ 186,596	$ 256,309	891,581	[3]	888,467	[3]	762,229	[3]
Cost of revenue
Total cost of revenue	[3]												301,765		279,673		239,256
Gross profit			178,446		128,840	122,962	159,568	180,628		121,226	118,721	188,219	589,816	[3]	608,794	[3]	522,973	[3]
Operating expenses
Selling and marketing	[3]												373,495		300,578		277,927
Research and development	[3]												181,710		162,886		145,548
General and administrative	[3]												51,643		52,153		45,951
Acquisition-related	[3]												0		0		2,903
Total operating expenses	[3]												606,848		515,617		472,329
(Loss) income from operations			15,453		(17,258)	(23,163)	7,936	38,518		(2,301)	(8,947)	65,907	(17,032)	[3]	93,177	[3]	50,644	[3]
Foreign currency transaction gain (loss)	[3],[4]												2,421		(6,413)		9,360
Interest income, net	[3]												2,705		862		911
Other income (expense), net	[3]												363		(1,391)		(5,580)
(Loss) income before (benefit) provision from income taxes	[3]												(11,543)		86,235		55,335
(Benefit) provision from income taxes	[3]												(22,160)		(12,313)		10,320
Net income			$ 16,413		$ (7,587)	$ (10,409)	$ 12,200	$ 40,595		$ 1,288	$ 3,702	$ 52,963	$ 10,617	[2],[3],[4],[5]	$ 98,548	[2],[3],[4],[5]	$ 45,015	[2],[3],[4],[5]
Earnings per share:
Basic (dollars per share)			$ 0.21		$ (0.10)	$ (0.13)	$ 0.16	$ 0.52		$ 0.01	$ 0.05	$ 0.69	$ 0.14	[3]	$ 1.27	[3]	$ 0.59	[3]
Diluted (dollars per share)			$ 0.20		$ (0.10)	$ (0.13)	$ 0.15	$ 0.49		$ 0.01	$ 0.04	$ 0.65	$ 0.13	[3]	$ 1.19	[3]	$ 0.56	[3]
Weighted-average number of common shares outstanding:
Basic (shares)	[3]												78,564		77,431		76,343
Diluted (shares)	[3]												83,064		82,832		79,732
Cumulative-effect adjustment to increase retained earnings		$ 204,100															$ 321	[2]
Previously Reported | Accounting Standards Update 2014-09
Assets
Accounts receivable, unbilled receivables, and contract assets								$ 248,331							$ 248,331
Long-term unbilled receivables								0							0
Deferred income taxes								57,127							57,127
Deferred contract costs								0							0
Other assets								416,148							416,148
Total assets								721,606							721,606
Liabilities and Stockholders’ Equity
Deferred revenue								195,073							195,073
Long-term deferred revenue								6,591							6,591
Deferred income tax liabilities								0							0
Other liabilities								148,864							148,864
Total liabilities								350,528							350,528
Foreign currency translation adjustments								(3,494)							(3,494)
Retained earnings								221,926							221,926
Other equity								152,646							152,646
Total stockholders’ equity								371,078							371,078
Total liabilities and stockholders’ equity								721,606							721,606
Revenue
Total revenue															840,582		750,266
Cost of revenue
Total cost of revenue															279,673		239,256
Gross profit															560,909		511,010
Operating expenses
Selling and marketing															307,210		278,849
Research and development															162,886		145,548
General and administrative															52,153		45,951
Acquisition-related															0		2,903
Total operating expenses															522,249		473,251
(Loss) income from operations															38,660		37,759
Foreign currency transaction gain (loss)															(900)		2,247
Interest income, net															731		776
Other income (expense), net															(1,391)		(5,580)
(Loss) income before (benefit) provision from income taxes															37,100		35,202
(Benefit) provision from income taxes															4,166		8,216
Net income															$ 32,934		$ 26,986
Earnings per share:
Basic (dollars per share)															$ 0.43		$ 0.35
Diluted (dollars per share)															$ 0.40		$ 0.34
Weighted-average number of common shares outstanding:
Basic (shares)															77,431		76,343
Diluted (shares)															82,832		79,732
Adjustments | Accounting Standards Update 2014-09
Assets
Accounts receivable, unbilled receivables, and contract assets								135,402							$ 135,402
Long-term unbilled receivables								160,708							160,708
Deferred income taxes								(42,887)							(42,887)
Deferred contract costs								37,924							37,924
Other assets								0							0
Total assets								291,147							291,147
Liabilities and Stockholders’ Equity
Deferred revenue								(29,223)							(29,223)
Long-term deferred revenue								(2,885)							(2,885)
Deferred income tax liabilities								38,463							38,463
Other liabilities								0							0
Total liabilities								6,355							6,355
Foreign currency translation adjustments								(2,979)							(2,979)
Retained earnings								287,771							287,771
Other equity								0							0
Total stockholders’ equity								284,792							284,792
Total liabilities and stockholders’ equity								$ 291,147							291,147
Revenue
Total revenue															47,885		$ 11,963
Cost of revenue
Total cost of revenue															0		0
Gross profit															47,885		11,963
Operating expenses
Selling and marketing															(6,632)		(922)
Research and development															0		0
General and administrative															0		0
Acquisition-related															0		0
Total operating expenses															(6,632)		(922)
(Loss) income from operations															54,517		12,885
Foreign currency transaction gain (loss)															(5,513)		7,113
Interest income, net															131		135
Other income (expense), net															0		0
(Loss) income before (benefit) provision from income taxes															49,135		20,133
(Benefit) provision from income taxes															(16,479)		2,104
Net income															65,614		18,029
Software license
Revenue
Total revenue	[3]												$ 288,119		339,294		297,284
Cost of revenue
Total cost of revenue	[3]												5,169		5,085		4,943
Software license | Previously Reported | Accounting Standards Update 2014-09
Revenue
Total revenue															288,334		279,995
Cost of revenue
Total cost of revenue															5,085		4,943
Software license | Adjustments | Accounting Standards Update 2014-09
Revenue
Total revenue															50,960		17,289
Cost of revenue
Total cost of revenue															0		0
Maintenance
Revenue
Total revenue	[3]												263,875		242,320		218,635
Cost of revenue
Total cost of revenue	[3]												24,565		27,905		25,505
Maintenance | Previously Reported | Accounting Standards Update 2014-09
Revenue
Total revenue															244,347		220,336
Cost of revenue
Total cost of revenue															27,905		25,505
Maintenance | Adjustments | Accounting Standards Update 2014-09
Revenue
Total revenue															(2,027)		(1,701)
Cost of revenue
Total cost of revenue															0		0
Services
Revenue
Total revenue	[3]												339,587		306,853		246,310
Cost of revenue
Total cost of revenue	[3]												$ 272,031		246,683		208,808
Services | Previously Reported | Accounting Standards Update 2014-09
Revenue
Total revenue															307,901		249,935
Cost of revenue
Total cost of revenue															246,683		208,808
Services | Adjustments | Accounting Standards Update 2014-09
Revenue
Total revenue															(1,048)		(3,625)
Cost of revenue
Total cost of revenue															$ 0		$ 0

[1] On January 1, 2018, the Company adopted the ASC 606 revenue recognition standard and adjusted prior periods to conform. See "Note 2. Significant Accounting Policies" for additional information.

[2] On January 1, 2018, the Company adopted the ASC 606 revenue recognition standard and adjusted prior periods to conform. See "Note 2. Significant Accounting Policies" for additional information.

[3] On January 1, 2018, the Company adopted the ASC 606 revenue recognition standard and adjusted prior periods to conform. See "Note 2. Significant Accounting Policies" for additional information.

[4] On January 1, 2018, the Company adopted the ASC 606 revenue recognition standard and adjusted prior periods to conform. See "Note 2. Significant Accounting Policies" for additional information.

[5] On January 1, 2018, the Company adopted the ASC 606 revenue recognition standard and adjusted prior periods to conform. See "Note 2. Significant Accounting Policies" for additional information.