Investments - Investments in Continuous Unrealized Loss Position for More Than Twelve Months (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 21,476	$ 24,196
Unrealized Losses	(65)	(20)
US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	1,994	0
Unrealized Losses	(6)	0
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	19,482	24,196
Unrealized Losses	$ (59)	$ (20)