Derivative Instruments (Additional Information) (Detail) (USD $)	9 Months Ended		24 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2011 Interest Rate Exchange Agreement Expiring 2010 and 2011	Dec. 06, 2010 4.5% Senior Notes, due December 2020	Dec. 09, 2011 4.2% Senior Notes, due December 2021	Dec. 31, 2011 Interest Rate Contract instrument
Derivative [Line Items]
Number of instruments held						2
Interest rate on senior notes				4.50%	4.20%
Amortization of interest expense	$ 1,290,000	$ 1,275,000	$ 68,900,000
Term of amortized interest expense, years			10 years
Amount to be recognized from hedged transactions within 12 months, approximate	$ 7,100,000