Debt (Narrative) (Details) - USD ($)	3 Months Ended			6 Months Ended	9 Months Ended
	May 31, 2018	Feb. 28, 2018	May 31, 2017	Feb. 28, 2018	May 31, 2018	May 31, 2017	Feb. 01, 2018	Aug. 31, 2017
Debt Instrument [Line Items]
Pro rata prepayment					$ 171,000,000	$ 10,417,000
Debt origination costs, net	$ 1,299,000				1,299,000			$ 2,439,000
Loss from debt transactions	0		$ 0	$ 1,300,000	1,310,000	$ 0
Prepayment premium		$ 200,000		200,000
Total assets	545,546,000				545,546,000			$ 561,744,000
Co-Issuers And Guarantor [Member]
Debt Instrument [Line Items]
Total assets	255,200,000				255,200,000
Restricted cash	$ 28,500,000				$ 28,500,000
Class A-2 2013 and 2016 Fixed Rate Notes [Member]
Debt Instrument [Line Items]
Pro rata prepayment		28,000,000		28,000,000
Write-off of unamortized debt origination costs		400,000		400,000
Class A-2 2018 Fixed Rate Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount							$ 170,000,000
Senior secured notes interest rate, percentage	4.03%				4.03%
Expected life of debt instrument					7 years
Debt instrument, maturity year					Feb. 20, 2025
Accrued interest	$ 170,200,000				$ 170,200,000
Weighted-average interest cost, percentage	4.40%				4.40%
Debt origination costs, net		5,000,000		5,000,000
Legal final maturity date					Feb. 20, 2048
2016 Variable Funding Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount							$ 100,000,000
Write-off of unamortized debt origination costs		$ 700,000		$ 700,000
Interest Rate Upward Adjustment Due To Nonpayment [Member]
Debt Instrument [Line Items]
Senior secured notes interest rate, percentage	5.00%				5.00%