Derivatives - Fair Values of Derivatives Reported on Condensed Consolidated Statements of Financial Condition (Parenthetical) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Gross derivative liability	$ 175,750	$ 170,388	$ 175,270	$ 193,574
Total Return Swaps [Member]
Derivatives, Fair Value [Line Items]
Gross derivative assets	509	357
Gross derivative liability	9,561	9,400
Cash collateral pledged for total return swaps	$ 17,496	$ 16,996