Borrowings (Details) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 USD ($)	Mar. 31, 2009 USD ($)	Feb. 28, 2009 USD ($)	Feb. 28, 2009 Letter of Credit [Member] USD ($)	Jun. 30, 2011 Standby Letters of Credit [Member] GBP (£)
Debt Instrument [Line Items]
Term of senior secured revolving credit facility			1 year
Amount that can be borrowed under the amended agreement		$ 250.0	$ 180.0	$ 10.0
Letters of credit outstanding, amount					1.2
Line of credit facility, interest rate description	(i) the federal funds rate plus a margin equal to 0.50%, (ii) Bank of America’s prime rate and (iii) the London Interbank Offered Rate (“LIBOR”) plus a margin equal to 1.00%); or LIBOR plus a margin equal to 0.60%, in each case for interest periods of 1, 2, 3 or 6 months as selected by the Company
Total principal amount outstanding under the revolving credit facility	$ 248.0
Approximate Interest rate payable on outstanding amount	0.84%
Margin rate under condition one of base rate	0.50%
Margin rate under condition three of base rate	1.00%
Margin rate above LIBOR under option two	0.60%