Summary of Significant Accounting Policies - Long-Lived Assets (Details)	12 Months Ended
	Dec. 31, 2017
Buildings | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	25 years
Buildings | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	33 years
Computer equipment | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	2 years
Computer equipment | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	5 years
Office furniture and other equipment. | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	2 years
Office furniture and other equipment. | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	5 years