Fair Value Measurements - Changes in Level 3 Assets Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance at the beginning of the year	$ 21,124	$ 28,459
Purchases	1,096	609
Settlements	(191)	(2,327)
Sales	(429)	(8,817)
(Losses) gains included in earnings	(111)	7,141
Total unrealized losses	(104)	(3,941)
Balance at the end of the period	$ 21,385	$ 21,124