Collateralized Transactions - Schedule of Company Lines Available and Borrowings Outstanding (Parenthetical) (Detail)			9 Months Ended	12 Months Ended
	Oct. 18, 2017	Oct. 11, 2017	Sep. 30, 2017	Dec. 31, 2016
Warehouse Line Due June 20, 2018 [Member]
Securities Financing Transaction [Line Items]
Maturity date			Jun. 20, 2018
Warehouse Line Due October 12, 2017 [Member]
Securities Financing Transaction [Line Items]
Maturity date			Oct. 12, 2017	Oct. 12, 2017
Warehouse Line Due September 25, 2018 [Member]
Securities Financing Transaction [Line Items]
Maturity date			Sep. 25, 2018
Warehouse Line Due April 21, 2017 [Member]
Securities Financing Transaction [Line Items]
Maturity date				Apr. 21, 2017
Warehouse Line Due September 25, 2017 [Member]
Securities Financing Transaction [Line Items]
Maturity date				Sep. 25, 2017
Subsequent Event | Warehouse Line Due June 20, 2018 [Member]
Securities Financing Transaction [Line Items]
Stated spread interest rate	1.30%
Subsequent Event | Warehouse Line Due October 12, 2017 [Member]
Securities Financing Transaction [Line Items]
Stated spread interest rate		1.30%
Extended maturity date		Oct. 11, 2018