GOODWILL & INTANGIBLE ASSETS - Narrative (Details) $ in Thousands	6 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) reporting_unit	Sep. 30, 2019 reporting_unit	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)
Acquired Finite-Lived Intangible Assets [Line Items]
Number of reporting units | reporting_unit	1	2
Goodwill	$ 1,725,680		$ 1,725,680	$ 1,715,485	$ 1,712,764
Carrying value of intangible assets	582,179		582,179	669,118
Impairment of intangible assets			$ 0	35,871	$ 0
Weighted average useful life of acquired intangible assets			14 years 7 months 6 days
Trade name
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets	$ 18,600		$ 18,600	18,600
Developed and core technology
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average useful life of acquired intangible assets			11 years 2 months 12 days
Customer and distributor relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted average useful life of acquired intangible assets			15 years 10 months 24 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)