Stock-Based Compensation - Schedule of Information Regarding Standard and Market Condition Based Stock Options Outstanding (Detail) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Standard Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock options outstanding (in shares)	3,604,069	3,486,157	3,227,167
Options vested and expected to vest using estimated forfeiture rates, number (in shares)	3,437,670	3,319,308
Options exercisable, number (in shares)	2,164,658	2,023,024
Options outstanding, Weighted Average Exercise Price (in dollars per share)	$ 8.42	$ 8.30
Options vested and expected to vest using estimated forfeiture rates, Weighted Average Exercise Price (in dollars per share)	8.33	8.21
Options exercisable, Weighted Average Exercise Price (in dollars per share)	$ 7.53	$ 7.18
Options outstanding, Weighted Average Remaining Contractual Life (years)	4 years 5 months 4 days	4 years 10 months 6 days
Options vested and expected to vest using estimated forfeiture rates, Weighted Average Remaining Contractual Life (years)	4 years 4 months 6 days	4 years 9 months 18 days
Options exercisable, weighted average remaining contractual (years)	3 years 8 months 4 days	4 years 3 months 4 days
Options outstanding, aggregate intrinsic value	$ 11.4	$ 6.6
Options vested and expected to vest using estimated forfeiture rates, Aggregate Intrinsic Value	11.1	6.6
Options exercisable, aggregate intrinsic value	$ 8.9	$ 6.0
Market Condition Based Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock options outstanding (in shares)	200,000	50,000	0
Options vested and expected to vest using estimated forfeiture rates, number (in shares)	180,300	45,430
Options exercisable, number (in shares)	0	0
Options outstanding, Weighted Average Exercise Price (in dollars per share)	$ 9.05	$ 11.94
Options vested and expected to vest using estimated forfeiture rates, Weighted Average Exercise Price (in dollars per share)	9.12	11.94
Options exercisable, Weighted Average Exercise Price (in dollars per share)	$ 0.00	$ 0.00
Options outstanding, Weighted Average Remaining Contractual Life (years)	6 years 2 months 1 day	6 years 1 month 24 days
Options vested and expected to vest using estimated forfeiture rates, Weighted Average Remaining Contractual Life (years)	6 years 1 month 24 days	6 years 1 month 24 days
Options outstanding, aggregate intrinsic value	$ 0.5	$ 0.0
Options vested and expected to vest using estimated forfeiture rates, Aggregate Intrinsic Value	0.4	0.0
Options exercisable, aggregate intrinsic value	$ 0.0	$ 0.0