Overview and Summary of Significant Accounting Policies - Estimated Useful Lives of Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2017
Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	3 years
Software [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	2 years
Software [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	3 years
Building and Building Improvements [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	7 years
Building and Building Improvements [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	39 years
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	2 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of property and equipment	Shorter of useful life (generally up to 7 years) or remaining lease term