Stock-Based Compensation - Schedule of Information Regarding Restricted Stock Units Outstanding (Detail) - RSUs outstanding - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs outstanding, number of Shares	508,880	427,192	487,423	564,891
RSUs vested and expected to vest using estimated forfeiture rates, number of shares	414,847	349,759	414,934
RSUs outstanding, weighted average remaining contractual life (years)	1 year 3 months 18 days	11 months 4 days	10 months 24 days
RSUs vested and expected to vest using estimated forfeiture rates, weighted average remaining contractual life (years)	1 year 1 month 27 days	9 months 18 days	10 months 13 days
RSUs outstanding, aggregate intrinsic value	$ 3.6	$ 4.5	$ 5.7
RSUs vested and expected to vest using estimated forfeiture rates, aggregate intrinsic value	2.9	3.7	4.8
RSUs outstanding, fair value	$ 3.6	$ 4.5	$ 5.7