Financial Instruments - Information Regarding Swaps (Detail) (USD $) In Millions, unless otherwise specified			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Interest Rate Swaps One [Member]	Dec. 31, 2012 Interest Rate Swaps Two [Member]	Dec. 31, 2012 Interest Rate Swaps Three [Member]	Dec. 31, 2012 Interest Rate Swaps Four [Member]	Dec. 31, 2012 Interest Rate Swaps Five [Member]	Dec. 31, 2012 Interest Rate Swaps Six [Member]	Dec. 31, 2012 Interest Rate Swap Seven [Member]
Derivative [Line Items]
Notional amount of interest rate derivatives	$ 632.8	$ 284.1	$ 100.0	$ 50.0	$ 100.0	$ 82.8	$ 100.0	$ 100.0	$ 100.0
LIBOR Fixed Rate			1.00%	0.60%	0.50%	2.80%	0.90%	1.60%	1.50%
Maturity Date			Jun. 01, 2014	Jun. 01, 2015	Jul. 01, 2015	Sep. 01, 2017	Jan. 01, 2018	Feb. 01, 2019	Feb. 01, 2019