Credit Facility - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended		1 Months Ended
	Apr. 30, 2012	Apr. 30, 2015	Apr. 30, 2014	Dec. 31, 2014	Oct. 31, 2014		Feb. 17, 2012
Debt Instrument [Line Items]
Senior unsecured revolving credit facility entered date	Feb. 17, 2012
Credit facility maximum borrowing capacity		$ 350,000,000					$ 350,000,000.0
Senior unsecured term loan facility, face amount							150,000,000.0
Payment schedule for term loan		Principal payments on a portion of the Term Loan are due in equal quarterly installments of $7.5 million, with the remainder due when the Credit Agreement expires in October 2016.
Senior unsecured term loan facility additional borrowings		150,000,000.0
Debt instrument quarterly principal payment		7,500,000
Proceeds from credit facility		250,000,000	200,000,000
Principal payments made during period		105,424,000	15,497,000
Senior unsecured term loan facility		60,000,000			75,000,000
Senior unsecured term loan facility long term		30,000,000			45,000,000	[1]
Revolver
Debt Instrument [Line Items]
Proceeds from credit facility				250,000,000
Principal payments made during period		90,000,000
Amount outstanding		160,000,000			0
Borrowings, interest rate		0.975%
Revolver | Minimum
Debt Instrument [Line Items]
Commitment fees percentage		0.15%
Revolver | Maximum
Debt Instrument [Line Items]
Commitment fees percentage		0.30%
Term Loan
Debt Instrument [Line Items]
Principal payments made during period		$ 15,000,000
Borrowings, interest rate		1.125%

[1] Derived from audited financial statements.