Management Contracts Purchased - Additional Information (Detail)	Jan. 12, 2018 USD ($) Funds	Dec. 01, 2017 USD ($)	Dec. 31, 2017 USD ($) Funds	Sep. 30, 2017 USD ($)
Contracts In Progress Costs And Earnings [Line Items]
Number of mutual funds | Funds			27
Management contracts impairment amount			$ 0	$ 0
Subsequent Event [Member]
Contracts In Progress Costs And Earnings [Line Items]
Number of mutual funds | Funds	28
Manning and Napier Group LLC and Rainier Investment Management LLC [Member]
Contracts In Progress Costs And Earnings [Line Items]
Business combination, assets acquired and liabilities assumed, assets		$ 122,000,000
Business combination, agreement date		May 10, 2017
Business combination, cash paid		$ 1,000,000
Business acquisition, effective date of acquisition		Dec. 01, 2017
Manning and Napier Group LLC and Rainier Investment Management LLC [Member] | Subsequent Event [Member]
Contracts In Progress Costs And Earnings [Line Items]
Business combination, assets acquired and liabilities assumed, assets	$ 253,000,000
Business combination, agreement date	May 10, 2017
Business combination, cash paid	$ 2,100,000
Business acquisition, effective date of acquisition	Jan. 12, 2018