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