Derivative Instruments - Additional Information (Detail) (USD $)	1 Months Ended	9 Months Ended		12 Months Ended					6 Months Ended
	Aug. 31, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2007 Agreement	Jun. 30, 2011	Sep. 30, 2012 Year 2013 [Member]	Jun. 30, 2011 6.50% notes due 2016 [Member]	Jun. 30, 2011 10.125% notes due 2015 [Member]	Jun. 30, 2011 Interest Rate Swap Agreement 1 [Member]	Jun. 30, 2011 Interest Rate Swap Agreement 2 [Member]
Derivative [Line Items]
Long-term notes issued				$ 325,000,000	$ 1,100,000,000		$ 500,000,000
Variable rate basis				3-month London Interbank Offered Rate (LIBOR)					6-month LIBOR	6-month LIBOR
Interest rate spread above London Interbank Offered Rate (LIBOR)				1.25%					4.05%	8.03%
Fixed interest rate under swap agreement				5.25%					6.50%	10.125%
Notional amount of interest rate swap agreement				325,000,000					500,000,000	150,000,000
Notional amount for forward starting interest rate swap agreements				1,500,000,000
Length of interest rate swap agreement (In years)				3 years
Proceeds from (payments for) settlement of interest rate swap agreements	25,382,000	0	(23,387,000)	89,777,000
Estimated amount of pretax loss in AOCI related to interest rate swap that would be reclassified to earnings within the next 12 months						5,567,000
Number of forward starting interest rate swap agreements				15
Forward component of the settlement	23,387,000
Accrued interest income	$ 1,995,000
Coupon rate of notes							6.50%	10.125%