GOODWILL & INTANGIBLE ASSETS - Narrative (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($)	Mar. 31, 2019 USD ($) reporting_unit	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)
Acquired Finite-Lived Intangible Assets [Line Items]
Number of reporting units | reporting_unit		2
Goodwill		$ 1,715,485	$ 1,712,764	$ 1,718,162
Carrying value of intangible assets		669,118	831,374
Impairment of intangible assets		$ 35,871	0	$ 0
Weighted average useful life of acquired intangible assets		14 years 7 months 2 days
Trade name
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets		$ 18,600	18,600
Technology license
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired			$ 500
Weighted average useful life of acquired intangible assets			3 years
Developed and core technology
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average useful life of acquired intangible assets		11 years 3 months
Customer and distributor relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average useful life of acquired intangible assets		15 years 10 months 18 days
Trademarks and trade names
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average useful life of acquired intangible assets		8 years 7 months 6 days
Capitalized software
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average useful life of acquired intangible assets		3 years
Handheld Network Test Tools Business | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Acquired Finite-Lived Intangible Assets [Line Items]
Increase (decrease) in intangible assets	$ (10,200)
Service Assurance reporting unit
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill		$ 1,200,000	$ 1,200,000
Security reporting unit
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill		$ 555,100	$ 555,900