Summary of Significant Accounting Policies - Summary of property and equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Furniture and fixtures | Minimum
Summary of property and equipment
Estimated useful lives	3 years
Furniture and fixtures | Maximum
Summary of property and equipment
Estimated useful lives	10 years
Computer software | Minimum
Summary of property and equipment
Estimated useful lives	1 year
Computer software | Maximum
Summary of property and equipment
Estimated useful lives	10 years
Data processing equipment | Minimum
Summary of property and equipment
Estimated useful lives	1 year
Data processing equipment | Maximum
Summary of property and equipment
Estimated useful lives	5 years
Buildings | Minimum
Summary of property and equipment
Estimated useful lives	1 year
Buildings | Maximum
Summary of property and equipment
Estimated useful lives	30 years
Other equipment | Minimum
Summary of property and equipment
Estimated useful lives	2 years
Other equipment | Maximum
Summary of property and equipment
Estimated useful lives	26 years
Leasehold improvements | Minimum
Summary of property and equipment
Estimated useful lives	1 year
Leasehold improvements | Maximum
Summary of property and equipment
Estimated useful lives	15 years
Software Developed for Internal Use
Summary of property and equipment
Internal costs capitalized	$ 6.4	$ 10.5
Software Developed for Internal Use | Minimum
Summary of property and equipment
Estimated useful lives	1 year
Software Developed for Internal Use | Maximum
Summary of property and equipment
Estimated useful lives	10 years