Revolving Line of Credit - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Mar. 31, 2012
Line of Credit Facility [Line Items]
Unsecured revolving line of credit	$ 200
Revolving credit facility, expiration date	2016-09-28
Revolving credit facility, interest rate description	Interest on amounts borrowed under the credit facility is based on (i) a base rate, which is the greater of (a) the prime rate and (b) the Federal Funds rate plus 0.50% or (ii) LIBOR plus an applicable margin. The margin on LIBOR borrowings ranges from 1.000% to 1.625% and is determined based on our consolidated leverage ratio.
Margin on LIBOR borrowings, minimum	1.00%
Margin on LIBOR borrowings, maximum	1.625%
Revolving credit facility, federal funds rate	0.50%
Revolving credit facility, utilization fees, percentage of total credit facility commitment to exceed	50.00%
Credit facility restrictive covenant, maximum consolidated leverage ratio	3.0
Credit facility restrictive covenant, minimum fixed charge ratio	2.5