Stock-Based Compensation - Schedule of Information Regarding Restricted Stock Units Outstanding (Detail) (Restricted Stock Units [Member], USD $) In Millions, except Share data, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2012
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs outstanding, Number of Shares	543,391	668,056	708,651
RSUs vested and expected to vest using estimated forfeiture rates, Number of Shares	471,343	583,711
RSUs outstanding, Weighted Average Remaining Contractual Life (years)	11 months 12 days	10 months 28 days
RSUs vested and expected to vest using estimated forfeiture rates, Weighted Average Remaining Contractual Life (years)	10 months 28 days	10 months 21 days
RSUs outstanding, Aggregate Intrinsic Value	$ 4.7	$ 6.9
RSUs vested and expected to vest using estimated forfeiture rates, Aggregate Intrinsic Value	4.0	6.1
RSUs outstanding, Fair Value	4.7	6.9
RSUs vested and expected to vest using estimated forfeiture rates, Fair Value