Long-Term Debt - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2013 SecurityLoan	Nov. 08, 2011
Debt Disclosure [Abstract]
Maximum amount of the revolving credit facility		$ 40.0
Maturity date for the revolving credit facility	Sep. 03, 2015
Period for which LIBOR election for amount of debt made	One, two or three months at a time
Maximum number of individual LIBOR loans	5
Minimum amount required for LIBOR rate election	1.0
Outstanding borrowings under credit facility	$ 0

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Period Type:	duration

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(2))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount borrowed under the credit facility as of the balance sheet date.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Debt instrument description of variable rate basis period.

+ References

No definition available.

+ Details

Name:	usmo_DebtInstrumentDescriptionOfVariableRateBasisPeriod
Namespace Prefix:	usmo_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum number of individual LIBOR rate loan.

+ References

No definition available.

+ Details

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Namespace Prefix:	usmo_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Minimum amount required for LIBOR rate election.

+ References

No definition available.

+ Details

Name:	usmo_MinimumAmountRequiredForLiborRateElection
Namespace Prefix:	usmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration