Fair Value Measurements - Changes in Level 3 Assets Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation Calculation Roll Forward
Balance at the beginning of the year	$ 20,171	$ 21,385
Purchases	2,118	1,668
Settlements	(2,385)	(722)
Sales	(73)	(5,412)
Gains included in earnings	67	4,060
Total unrealized losses	54	(808)
Balance at the end of the period	$ 19,952	$ 20,171