Restructuring Costs Restructuring Liability (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Termination Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	$ 3.3	$ 0.0
Costs incurred and charged to expense	7.0
Costs paid or otherwise settled	(3.7)
Office lease Termination Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	0.0	0.0
Costs incurred and charged to expense	0.3
Costs paid or otherwise settled	(0.3)
Accelerated Share Based Compensation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	0.0	0.0
Costs incurred and charged to expense	4.0
Costs paid or otherwise settled	(4.0)
Retention Expense [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	8.0	0.0
Costs incurred and charged to expense	8.0
Costs paid or otherwise settled	0.0
Total [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	11.3	$ 0.0
Costs incurred and charged to expense	19.3
Costs paid or otherwise settled	$ (8.0)