Income Taxes (Narratives) (Details) - USD ($)	3 Months Ended			6 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2017	Jun. 30, 2018
Operating Loss Carryforwards [Line Items]
Provision for income taxes	$ 1,402,000		$ 1,675,000
Description of effective date of enacted tax rate implementation	Effective for tax years beginning on or after January 1, 2018.
U.S. Federal tax rate	21.00%			35.00%
Tax benefit		$ 2,500,000			$ 2,500,000
Additional tax provision resulting corresponding reduction in valuation allowance	$ 0
Unrecognized tax benefits	17,200,000
Unrecognized tax benefit future impact if recognized	$ 17,200,000
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	$ 500,000		$ 1,600,000
Deferred tax liability, goodwill	$ 5,900,000
Amortization period of intangible assets resulting from acquisitions	15 years
ASU 2016-16 | Ireland | Subsidiary
Operating Loss Carryforwards [Line Items]
Deferred tax asset relating to a transfer of certain assets to subsidiary	$ 5,400,000