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Restriction on Distribution of Net Assets from Subsidiary - Additional Information (Detail) - USD ($) | Jul. 10, 2018 | Jul. 05, 2018 | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | --- | --- | ---
Subsequent Event [Member] |  |  |  |
Financial Receivables Impaired Or Restructured [Line Items] |  |  |  |
Dividends received | $ 10,400,000 |  |  |
WMMRC [Member] |  |  |  |
Financial Receivables Impaired Or Restructured [Line Items] |  |  |  |
Total net assets |  |  | $ 12,400,000 | $ 13,100,000
WMMRC [Member] | Subsequent Event [Member] |  |  |  |
Financial Receivables Impaired Or Restructured [Line Items] |  |  |  |
Dividend declaration date | Jul. 10, 2018 |  |  |
WMMRC [Member] | Maximum [Member] | Subsequent Event [Member] |  |  |  |
Financial Receivables Impaired Or Restructured [Line Items] |  |  |  |
Net assets available for distribution from subsidairy |  | $ 11,100,000 |  |

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- Definition Date the dividend to be paid was declared, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 + Details
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- Definition Dividends received on equity and other investments during the current period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585 + Details
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