Collateralized Securities Transactions - Additional Information (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Disclosure of Repurchase Agreements [Abstract]
Securities purchased under agreements to resell, securities borrowed and margin agreements on terms that permit to repledge or resell the securities to others		$ 192,200,000
Securities either pledged or otherwise transferred to others in connection with financing activities or to satisfy commitments under financial instruments and other inventory positions sold, but not yet purchased		185,200,000
Securities loaned	$ 0	$ 0.00