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Fair Value Measurements (Reconciliation of Fair Value Using Significant Unobservable Inputs ) (Details) (Convertible Preferred Stock [Member], USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Convertible Preferred Stock [Member]
Beginning balance	$ 83.3		$ 78.2
Less: paid-in-kind dividends payable, beginning balance	(2.6)		(2.4)
Issuances	2.6	75.0	7.5	75.0
Paid-in-kind dividends payable	2.6	0.8	2.6	0.8
Ending balance	$ 85.9	$ 75.8	$ 85.9	$ 75.8

X

- Definition

Issues that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

This element represents a liability measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Paid in kind convertible preferred stock dividends payable

+ Details

Name:	usu_PaidInKindDividendsPayable
Namespace Prefix:	usu
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Paid in kind convertible preferred stock dividends payable beginning balance

+ Details

Name:	usu_PaidInKindDividendsPayableBeginningBalance
Namespace Prefix:	usu
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration