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Income Taxes - Summary of Valuation Allowance (Detail) - USD ($) $ in Millions | 12 Months Ended
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Jun. 30, 2021 | Jun. 30, 2020
Valuation Allowance [Abstract] |  |
Valuation allowance balance – beginning of fiscal year | $ 29.0 | $ 18.7
Increases resulting from business combinations | 0.5 | 0.3
Other increases | 8.6 | 10.0
Other decreases | (9.0) | 0.0
Valuation allowance balance – end of fiscal year | $ 29.1 | $ 29.0

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- Definition Valuation allowance deferred tax asset increases from business combination. + References No definition available. + Details
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Balance Type: | credit
Period Type: | duration

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- Definition Valuation allowance deferred tax asset other decreases. + References No definition available. + Details
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Namespace Prefix: | mir_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Valuation allowance deferred tax asset other increases. + References No definition available. + Details
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Namespace Prefix: | mir_
Data Type: | xbrli:monetaryItemType
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- Definition Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121826272&loc=d3e32537-109319 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Period Type: | duration