Restriction on Distribution of Net Assets from Subsidiary - Additional Information (Detail) (WMMRC [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
WMMRC [Member]
Financial Receivables Impaired Or Restructured [Line Items]
Total net assets	$ 167.1	$ 167.0

The net of assets and liabilities.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 09

-Paragraph 19

-Article 6

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 946

-SubTopic 225

-Section S99

-Paragraph 3

-Subparagraph (SX 210.6-09.6,7)

-URI http://asc.fasb.org/extlink&oid=6488393&loc=d3e606914-122999

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 946

-SubTopic 210

-Section S99

-Paragraph 2

-Subparagraph (SX 210.6-05.4)

-URI http://asc.fasb.org/extlink&oid=6488278&loc=d3e604008-122996

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 05

-Paragraph 4

-Article 6

Name:	us-gaap_AssetsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant