Derivative Instruments - Interest Rate Derivative Positions Outstanding Table (Detail) (USD $)	Oct. 31, 2011	Jun. 30, 2009	Mar. 31, 2009	Dec. 31, 2011 Reference Period Start End Dates - October 2012 to October 2022 [Member]	Dec. 31, 2011 Reference Period Start End Dates - October 2012 to October 2042 [Member]	Dec. 31, 2011 Reference Period Start End Dates - June 2014 to June 2024 [Member]	Dec. 31, 2011 Reference Period Start End Dates - June 2014 to June 2044 [Member]
Derivative [Line Items]
Notional principal amount of interest-rate swap	$ 150,000,000			$ 250	$ 750	$ 750	$ 1,100
Weighted-average interest rate for interest-rate swap		4.80%	3.25%	4.91%	4.80%	6.00%	5.57%