Interest Rate Derivative Positions Outstanding Table (Detail) (USD $)	Dec. 31, 2009	Mar. 31, 2009	Sep. 30, 2011 Reference Period Start End Dates - October 2011 to October 2041 [Member]	Sep. 30, 2011 Reference Period Start End Dates - October 2012 to October 2022 [Member]	Sep. 30, 2011 Reference Period Start End Dates - October 2012 to October 2042 [Member]	Sep. 30, 2011 Reference Period Start End Dates - June 2014 to June 2024 [Member]	Sep. 30, 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	$ 250,000,000	$ 750,000,000	$ 750,000,000	$ 1,100,000,000
Weighted-average interest rate for interest-rate swap	4.80%	3.25%	4.65%	4.91%	4.80%	6.00%	5.57%