Financial Instruments - Gains (Losses) from Level 3 Financial Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Institutional Brokerage [Member]
Total gains and losses included in earnings for the period:
Included in earnings	$ 422	$ 38	$ 1,722
Change in unrealized gains or losses relating to assets still held at the end of the respective period	123	(38)	438
Net Investment (Loss) Income [Member]
Total gains and losses included in earnings for the period:
Included in earnings	(754)	(415)	2,648
Change in unrealized gains or losses relating to assets still held at the end of the respective period	$ (754)	$ 304	$ 2,551