Restructuring Costs Restructuring Liability (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
Restructuring Cost and Reserve [Line Items]
Balance at December 31, 2018	$ 30.3
Costs incurred and charged to expense	46.2
Costs paid or otherwise settled	(68.8)
Balance at December 31, 2019	7.7
Termination Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Balance at December 31, 2018	19.5
Costs incurred and charged to expense	12.3
Costs paid or otherwise settled	(30.6)
Balance at December 31, 2019	1.2
Office lease termination costs
Restructuring Cost and Reserve [Line Items]
Balance at December 31, 2018	0.0
Costs incurred and charged to expense	0.6
Costs paid or otherwise settled	(0.6)
Balance at December 31, 2019	0.0
Accelerated Share Based Compensation [Member]
Restructuring Cost and Reserve [Line Items]
Balance at December 31, 2018	0.0
Costs incurred and charged to expense	12.6
Costs paid or otherwise settled	(12.6)
Balance at December 31, 2019	0.0
Retention Expense [Member]
Restructuring Cost and Reserve [Line Items]
Balance at December 31, 2018	10.8
Costs incurred and charged to expense	19.5
Costs paid or otherwise settled	(23.8)
Balance at December 31, 2019	6.5
Pension and Medical Plan curtailment
Restructuring Cost and Reserve [Line Items]
Balance at December 31, 2018	0.0
Costs incurred and charged to expense	1.2
Costs paid or otherwise settled	(1.2)
Balance at December 31, 2019	$ 0.0