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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 |  |  |  |

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