Summary of Operations and Significant Accounting Policies - Summary of Property, Equipment and Software Over Estimated Useful Life (Detail)	12 Months Ended
	Dec. 31, 2017
Buildings [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	39 years
Office Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	7 years
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Leasehold improvements	Shorter of lease term or estimated useful life
Minimum [Member] | Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Maximum [Member] | Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	10 years