Debt (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Percentage of outstanding debt priced at variable interest rates	100.00%
Amount of debt hedged	$ 435,000,000
Number of interest rate swaps hedging long-term debt	3
Weighted average effective borrowing rate	4.70%
Amount of debt unhedged	77,200,000
Maximum basis-point change that would not affect annual interest expense	50
Receivables sold to Investor	418,100,000
Credit facility	50,000,000
Basis spread on variable rate	1.30%
2007 Note Purchase Agreement [Member]
Debt Instrument [Line Items]
Leverage ratio	3.25
2011 Credit Agreement [Member]
Debt Instrument [Line Items]
Debt instrument maturity period, in years	5 years
Credit facility with aggregate committed principal amount	700,000,000
Potential maximum committed principal amount	1,000,000,000
Senior Secured Notes [Member]
Debt Instrument [Line Items]
Issuance of senior secured notes	1,000,000,000
Senior secured notes	135,000,000
Maturity date of debt instrument	Oct. 15, 2014
Receivables Securitization Program [Member]
Debt Instrument [Line Items]
Maximum investment at one time outstanding	100,000,000
Leverage ratio	3.25
Receivables sold to Investor	418,100,000	421,000,000
Amounts borrowed by USR		0
Receivables Securitization Program [Member] | 2011 Credit Agreement [Member]
Debt Instrument [Line Items]
Outside indebtedness permitted under facility	200,000,000
Prior Receivables Securitization Program [Member]
Debt Instrument [Line Items]
Receivables securitization program, expiration date	Jan. 20, 2012
Letters Of Credit [Member] | 2011 Credit Agreement [Member]
Debt Instrument [Line Items]
Credit facility with aggregate committed principal amount	100,000,000
Outstanding letters of credit	10,200,000	10,300,000
Swing Line Loans [Member] | 2011 Credit Agreement [Member]
Debt Instrument [Line Items]
Letter of credit maximum capacity	50,000,000
Unspecified [Member] | 2011 Credit Agreement [Member]
Debt Instrument [Line Items]
Outside indebtedness permitted under facility	300,000,000
Note Purchase Agreement Replacement [Member] | 2011 Credit Agreement [Member]
Debt Instrument [Line Items]
Outside indebtedness permitted under facility	$ 135,000,000
Minimum [Member] | 2007 Note Purchase Agreement [Member]
Debt Instrument [Line Items]
Leverage ratio	2.75
Minimum [Member] | 2011 Credit Agreement [Member]
Debt Instrument [Line Items]
Leverage ratio	3.00
Maximum [Member] | 2011 Credit Agreement [Member]
Debt Instrument [Line Items]
Leverage ratio	3.50