Derivative Instruments - Additional Information (Detail) (USD $)	1 Months Ended	9 Months Ended			1 Months Ended			1 Months Ended			1 Months Ended
	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 29, 2011	Dec. 31, 2010 Interest Rate Exchange Agreement Expiring 2010 Y	Apr. 30, 2010 Interest Rate Exchange Agreement Expiring 2010	Apr. 15, 2010 Interest Rate Exchange Agreement Expiring 2010	Oct. 31, 2011 Interest Rate Exchange Agreement Expiring 2011	Jul. 31, 2011 Interest Rate Exchange Agreement Expiring 2011	Jul. 12, 2011 Interest Rate Exchange Agreement Expiring 2011	Dec. 31, 2011 Interest Rate Exchange Agreements Expiring 2011 And 2012	Dec. 06, 2010 4.5 % Senior Notes	Apr. 15, 2010 4.5 % Senior Notes
Derivative [Line Items]
Interest rate contract with notional amount				$ 350,000,000			$ 300,000,000			$ 350,000,000
Interest rate exchange agreement expiration date	Feb. 28, 2012					Dec. 31, 2010			Sep. 30, 2011
Interest rate on senior notes												4.50%	4.50%
Interest rate contract settlement amount					31,000,000			34,700,000
Amortization interest expense		1,109,000	947,000		31,000,000						37,900,000
Term of amortized interest expense, years					10
Interest rate					5.80%						5.30%
Settlement amount interest rate contract											38,700,000
Amount to be recognized from hedged transactions within 12 months, approximate		$ 7,500,000