Stock-Based Compensation - Schedule of Information Regarding Restricted Stock Units Outstanding (Detail) - Restricted Stock Units - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs outstanding, number (in shares)	516,956	564,891	668,056
RSUs vested and expected to vest using estimated forfeiture rates, number (in shares)	425,164	502,411
RSUs outstanding, Weighted Average Remaining Contractual Life (years)	1 year 1 month 6 days	10 months 2 days
RSUs vested and expected to vest using estimated forfeiture rates, Weighted Average Remaining Contractual Life (years)	1 year 21 days	9 months 18 days
RSUs outstanding, Aggregate Intrinsic Value	$ 5.8	$ 5.3
RSUs vested and expected to vest using estimated forfeiture rates, Aggregate Intrinsic Value	$ 4.8	$ 4.8