Summary of Significant Accounting Policies - Capitalized Software and Hardware (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment
Amortization expense	$ 2.8	$ 3.1	$ 2.4
Prepaid Expenses and Other Current Assets
Property, Plant and Equipment
Capitalized internal use software and hardware	$ 6.0	$ 3.3
Minimum
Property, Plant and Equipment
Amortization period	1 year
Maximum
Property, Plant and Equipment
Amortization period	5 years