Debt - Additional Information (Detail) (USD $) In Millions, unless otherwise specified			12 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Senior Notes [Member]	Dec. 01, 2009 Senior Notes [Member]	Dec. 31, 2012 Revolving credit facility [Member]	Dec. 31, 2011 Revolving credit facility [Member]	Dec. 31, 2010 Revolving credit facility [Member]
Debt Instrument [Line Items]
Line of credit facility, borrowing capacity					$ 300.0
Line of credit facility, commitment fees					1.0	1.1	1.3
Debt instrument, unused borrowing capacity					290.0
Letters of credit, amount outstanding					10.0
Debt, Weighted Average Interest Rate			7.875%		0.00%
Debt instrument, issuance date			Dec. 01, 2009
Debt instrument, offering price percentage			98.311%
Debt instrument, offering value			294.9
Debt instrument, face amount				300.0
Debt instrument, effective interest rate			8.125%
Debt instrument, redemption price percentage			103.938%
Accumulated amortization	4.6	3.4
Unamortized deferred financing costs	$ 7.2	$ 8.2