Restructuring Costs Restructuring Liability (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Termination Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	$ 10.0	$ 19.5
Costs incurred and charged to expense	6.8
Costs paid or otherwise settled	(16.3)
Office lease Termination Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	0.6	0.0
Costs incurred and charged to expense	0.6
Costs paid or otherwise settled	0.0
Accelerated Share Based Compensation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	0.0	0.0
Costs incurred and charged to expense	8.4
Costs paid or otherwise settled	(8.4)
Retention Expense [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	1.4	10.8
Costs incurred and charged to expense	6.1
Costs paid or otherwise settled	(15.5)
Pension curtailment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	0.0	0.0
Costs incurred and charged to expense	(0.5)
Costs paid or otherwise settled	0.5
Total [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Liability	12.0	$ 30.3
Costs incurred and charged to expense	21.4
Costs paid or otherwise settled	$ (39.7)