Property and Equipment, Net (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2017
Office and Computer Equipment [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	3 years
Office and Computer Equipment [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	10 years
Office Furniture and Fixtures [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	3 years
Office Furniture and Fixtures [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	10 years
Internal-use Computer Software [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	1 year
Internal-use Computer Software [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	10 years
Leasehold Improvements and Building [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	2 years
Leasehold Improvements and Building [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	20 years