Summary of Significant Accounting Policies - Owned Properties (Details)	12 Months Ended
	Dec. 31, 2017
Computer Equipment | Minimum
Owned Properties
Useful life	3 years
Computer Equipment | Maximum
Owned Properties
Useful life	5 years
Furniture and Fixtures | Minimum
Owned Properties
Useful life	5 years
Furniture and Fixtures | Maximum
Owned Properties
Useful life	15 years
Building | Minimum
Owned Properties
Useful life	35 years
Building | Maximum
Owned Properties
Useful life	50 years
Building Improvements | Minimum
Owned Properties
Useful life	10 years
Building Improvements | Maximum
Owned Properties
Useful life	20 years