Summary of Valuation of Company's Derivatives by Pricing Observability Levels (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Total Carrying Amount
Assets
Derivatives	$ 1.9	[1]	$ 16.5
Total Assets	1.9		16.5
Liabilities
Derivatives	0.7		0.7
Total Liabilities	0.7		0.7
Fair Value, Inputs, Level 2
Assets
Derivatives	1.9	[1]	16.5
Total Assets	1.9		16.5
Liabilities
Derivatives	0.7		0.7
Total Liabilities	$ 0.7		$ 0.7

[1] The decrease from previous period is explained by the closure of a $60 million interest rate swap in Q1 of 2013.