Debt (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	9 Months Ended
	Dec. 31, 2010	Sep. 30, 2012	Dec. 31, 2011
Debt Instruments Maturity Duration	5 years
Line Of Credit Facility Additional Borrowing Capacity	$ 150.0	$ 354.2
Line of Credit Facility, Interest Rate Description	either (i) the greatest of: the administrative agent's prime rate; the average rate on overnight federal funds plus 1/2 of 1%; and the Eurodollar rate (adjusted for statutory reserves) plus 1%, in each case plus a margin equal to between 0.50% and 1.25% depending on the Company's leverage ratio as of the end of the four consecutive fiscal quarters most recently ended, or (ii) the Eurodollar rate (adjusted for statutory reserves) plus a margin equal to between 1.50% and 2.25%
Debt Instrument Interest Additional Interest Above Eurodollar Rate	1.00%
Debt Instrument Interest Additional Interest Above Federal Fund Rate	1.00%
Notional Amount of Interest Rate Derivatives		200.0
Derivative, Fixed Interest Rate		2.26%
Derivative Maturity Month And Year		September 2015
Interest Rate Fair Value Hedge Derivative at Fair Value, Net		11.0	9.9
Letters of Credit Outstanding, Amount		10.1
Letter of Credit [Member] | Revolver [Member]
Line of Credit Facility, Maximum Borrowing Capacity	40.0
Euro Dollar Rate [Member] | Federal Funds Rate [Member]
Minimum Applicable Margin Rate	0.50%
Maximum Applicable Margin Rate	1.25%
Euro Dollar Rate [Member]
Minimum Applicable Margin Rate	1.50%
Maximum Applicable Margin Rate	2.25%
Bank Term Loan [Member]
Line of Credit Facility, Maximum Borrowing Capacity	200.0
Line of Credit Facility, Frequency of Payments	19 consecutive quarterly installments
Debt Instrument, Maturity Date	Dec. 22, 2015
Revolver [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 400.0
Debt Instrument, Maturity Date	Dec. 22, 2015