Derivatives - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012
Derivative [Line Items]
Notional amount of interest rate derivatives	$ 888.4	$ 888.4
Amount reclassified to decrease to interest expense		0.4
Hedge	0	0
Termination value of derivatives in a net liability position	18.9	18.9
Collateral obligations for derivative counterparties	17.5	17.5
Interest rate swap [Member]
Derivative [Line Items]
Notional amount of interest rate derivatives	$ 140.0	$ 140.0