Income taxes - Additional Information (Detail) (USD $)	3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Valuation Allowance [Line Items]
Cumulative valuation allowance	$ 88,900,000
Net Deferred Tax Liabilities	(7,117,000)	(5,874,000)
Deferred tax liability, indefinite-lived intangible assets	7,117,000	5,874,000
Tax credit carryforward	3,200,000
Unrecognized Tax Benefits and Income Tax Penalties and Interest Accrued	72,000	72,000
Unrecognized tax benefits, which if recognized would impact on effective tax rate	50,000
Interest on income tax	20,000
Accrued interest and penalties from unrecognized tax positions	22,000	22,000
Federal
Valuation Allowance [Line Items]
Operating loss carryforwards	$ 124,500,000