Cash Equivalents and Investments (Narrative) (Detail) - USD ($)		3 Months Ended	9 Months Ended	12 Months Ended
		Sep. 30, 2016	Sep. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Unrealized Gain (Loss) on Investments	[1]		$ 648,000	$ 0
Gains (losses) on sales of available-for-sale securities		$ 0

[1] unaudited