Stock-based Compensation - Valuation Assumptions for Performance-based and Market-based RSUs - Additional Information (Detail) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted	0	0
Performance-based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted	0
Market-based RSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted	0
Market-based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted	0