Financial Instruments - Changes in Fair Value of Company's Level 3 Financial Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Trading Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 3,188	$ 11,535
Total net losses (realized/unrealized) included in earnings	(332)	(254)
Purchases	75,173	124,223
Sales/distributions	(61,495)	(126,067)
Transfers out of Level 3	(29)	(6,249)
Ending balance	16,505	3,188
The amount of total gains or losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date	(631)	266
Financial Instruments Sold, not yet Purchased [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 0	(1,499)
Total net losses (realized/unrealized) included in earnings		(122)
Purchases		6,043
Sales/distributions		(4,422)
Transfers out of Level 3		0
Ending balance		0
The amount of total gains or losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date		$ 0