Derivatives - Fair Values of Derivatives Reported on Consolidated Statements of Financial Condition (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Gross derivative liability	$ 207,306	$ 162,422	$ 97,593
Cash collateral pledged for total return swaps	12,364	11,384
Total Return Swaps [Member]
Derivatives, Fair Value [Line Items]
Gross derivative assets	1,123	2,019
Gross derivative liability	$ 1,786	$ 1,587