Management Contracts Purchased - Additional Information (Detail)	3 Months Ended
	Dec. 31, 2016 USD ($) Funds	Sep. 23, 2016 USD ($)	Dec. 31, 2015 USD ($)
Contracts In Progress Costs And Earnings [Line Items]
Number of mutual funds | Funds	25
Management contracts impairment amount	$ 0		$ 0
Westport Fund and Westport Select Cap Fund [Member]
Contracts In Progress Costs And Earnings [Line Items]
Acquisition of Westport Fund and the Westport Select Cap Fund		$ 435,000,000
Cash paid	11,300,000
Total capitalized costs related to purchase	$ 11,400,000