Restructuring Costs Restructuring Liability (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Termination Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	$ 5.3	$ 0.0
Costs incurred and charged to expense	13.7
Costs paid or otherwise settled	(8.4)
Office lease Termination Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	0.0	0.0
Costs incurred and charged to expense	1.0
Costs paid or otherwise settled	(1.0)
Accelerated Share Based Compensation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	0.0	0.0
Costs incurred and charged to expense	7.2
Costs paid or otherwise settled	(7.2)
Retention Expense [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	5.8	0.0
Costs incurred and charged to expense	13.8
Costs paid or otherwise settled	(8.0)
Pension curtailment [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)
Total [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	11.1	$ 0.0
Costs incurred and charged to expense	36.0
Costs paid or otherwise settled	$ (24.9)