Fair Value Measurements - Changes in Level 3 Assets Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance at the beginning of the year	$ 28,909	$ 32,002
Purchases, issuances and settlements	(1,070)	(1,518)
Sales	(8,593)	(813)
Gains included in earnings	7,021	673
Total unrealized (losses) gains	(4,029)	156
Balance at the end of the period	$ 22,238	$ 30,500