Commitments and Contingencies - Retirement Plans (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) employee	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of participants | employee	450
Corrpro
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions by employer, plan assets	$ 0.0	$ 0.1	$ 0.1
Benefit obligation	7.5	7.4
Fair value of plan assets	$ 9.3	8.8
Assumptions used calculating benefit obligation, discount rate	2.50%
Amounts for asset (liability) recognized in statement of financial position	$ 1.8
United States
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contribution plan cost	6.3	5.5	5.5
Foreign
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contribution plan cost	$ 1.0	$ 0.8	$ 0.8