Derivatives - Fair Values of Derivatives Reported on Condensed Consolidated Statements of Financial Condition (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Gross derivative liability	$ 212,416	$ 182,301	$ 174,740	$ 170,388
Total Return Swaps [Member]
Derivatives, Fair Value [Line Items]
Gross derivative assets	5,602	469
Gross derivative liability	398	11,321
Cash collateral pledged for total return swaps	$ 1,896	$ 17,616