Distributions to Owners and Related Activity (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Related Party Transaction [Line Items]
Beginning Balance	$ 563,984	$ 510,946
Cash advances to Perpetual	31,000	44,800
Repayment of cash advances from Perpetual
Charge for federal and state income taxes	(5,849)	(11,473)
Payment of income taxes	4,354	9,202
Ending Balance	593,489	553,475
Distributions To Owners and Dividends
Related Party Transaction [Line Items]
Beginning Balance	563,984	510,946
Cash advances to Perpetual	31,000	44,800
Repayment of cash advances from Perpetual
Ending Balance	594,984	555,746
Federal and State Income Tax Receivable (Payable)
Related Party Transaction [Line Items]
Beginning Balance
Charge for federal and state income taxes	(5,849)	(11,473)
Payment of income taxes	4,354	9,202
Ending Balance	$ (1,495)	$ (2,271)