Derivative Instruments - Additional Information (Detail) (USD $)	1 Months Ended		12 Months Ended				1 Months Ended			1 Months Ended			1 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	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
Future amortization interest expense over 10 year periods			1,685,000	1,263,000	547,000		31,000,000						37,900,000
Term of amortized interest expense, years							10
Interest rate							5.80%						5.30%
Additional interest rate contract settlement	4,000,000
Settlement amount interest rate contract													38,700,000
Interest rate settlement recognized as other expense													800,000
Amount to be recognized from hedged transactions within 12 months, approximate			$ 7,400,000