Income Taxes (Narratives) (Details) - USD ($)	3 Months Ended			6 Months Ended	9 Months Ended			12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2017	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018	Jun. 30, 2018	Dec. 31, 2018
Operating Loss Carryforwards [Line Items]
Income tax provision (benefit)	$ 1,877,000	$ 1,729,000			$ (1,991,000)	$ 1,800,000	$ 1,787,000
State net operating loss	0	$ 0			0		0
Tax benefit, foreign					2,600,000
Tax benefit			$ 2,500,000		$ 4,700,000		$ 2,500,000	$ 2,500,000
U.S. Federal tax rate				35.00%	21.00%		35.00%
Description of effective date of enacted tax rate implementation					Effective for tax years beginning on or after January 1, 2018.
Additional tax provision resulting corresponding reduction in valuation allowance									$ 0
Reduced valuation allowance	4,700,000				$ 4,700,000
Unrecognized tax benefits	17,400,000				17,400,000
Unrecognized tax benefit future impact if recognized	17,400,000				$ 17,400,000
State
Operating Loss Carryforwards [Line Items]
Income tax examination, years under examination					2014 2015 2016
Enterasys, Zebra WLAN Business, Campus Fabric Business and Brocade Data Center Business
Operating Loss Carryforwards [Line Items]
Tax amortization deduction for capitalized goodwill	1,800,000				$ 5,800,000
Deferred tax liability, goodwill	$ 6,800,000				$ 6,800,000
Amortization period of intangible assets resulting from acquisitions					15 years
Ireland | Subsidiary
Operating Loss Carryforwards [Line Items]
Amortization period of intangible assets resulting from acquisitions					6 years
ASU 2016-16 | Ireland | Subsidiary
Operating Loss Carryforwards [Line Items]
Deferred tax asset relating to a transfer of certain assets to subsidiary						$ 3,700,000