Restriction on Distribution of Net Assets from Subsidiary - Additional Information (Detail) (WMMRC [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
WMMRC [Member]
Financial Receivables Impaired Or Restructured [Line Items]
Total net assets	$ 54.9	$ 145.8	$ 167.0

Amount of net assets (liabilities).

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

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

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

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

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