Restriction on Distribution of Net Assets from Subsidiary - Additional Information (Detail) (WMMRC [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
WMMRC [Member]
Financial Receivables Impaired Or Restructured [Line Items]
Total net assets	$ 53.4	$ 54.9

Amount of net assets (liabilities).

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 946

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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