Summary of Significant Accounting Policies - Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2018
Building
Property and Equipment [Line Items]
Property and equipment, useful life (in years)	39 years
Minimum | Computer equipment and software
Property and Equipment [Line Items]
Property and equipment, useful life (in years)	3 years
Minimum | Furniture and fixtures
Property and Equipment [Line Items]
Property and equipment, useful life (in years)	3 years
Maximum | Computer equipment and software
Property and Equipment [Line Items]
Property and equipment, useful life (in years)	5 years
Maximum | Furniture and fixtures
Property and Equipment [Line Items]
Property and equipment, useful life (in years)	7 years