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Subsequent Events - Additional Information (Detail) - USD ($) | Jul. 10, 2018 | Jul. 13, 2018 | Jul. 05, 2018 | Jun. 30, 2018 | Dec. 31, 2017 | Jan. 05, 2015
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Subsequent Event [Line Items] |  |  |  |  |  |
Offering amount deposited into escrow |  |  |  | $ 575,800,000 | $ 578,900,000 | $ 598,500,000
Subsequent Event [Member] |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |
Dividends received | $ 10,400,000 |  |  |  |  |
Offering amount deposited into escrow |  | $ 1,700,000,000 |  |  |  |
Subsequent Event [Member] | WMMRC [Member] |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |
Dividend declaration date | Jul. 10, 2018 |  |  |  |  |
Subsequent Event [Member] | WMMRC [Member] | Maximum [Member] |  |  |  |  |  |
Subsequent Event [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 The designation of funds furnished by a borrower to a lender to assure future payments of the borrower's real estate taxes and insurance obligations with respect to a mortgaged property. Escrow deposits may be made for a variety of other purposes such as earnest money and contingent payments. This element excludes replacement reserves which are an escrow separately provided for within the US GAAP taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.10) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + 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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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
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- Definition Approval from insurance division to transfer of net asets from subsidairy. + References No definition available. + Details
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- Details
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- Details
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- Details
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