Restructuring Costs Restructuring Expense Table (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accelerated share-based compensation(1)	$ 8.4	$ 2.8
Other Expense [Member]
Total restructuring costs	(0.3)	0.0
Termination Benefits [Member]
Costs recognized period from inception to March 31, 2019(1)	39.0
Total remaining costs expected to be incurred	0.0
Office lease Termination Costs [Member]
Costs recognized period from inception to March 31, 2019(1)	1.6
Total remaining costs expected to be incurred	0.0
Accelerated Share Based Compensation [Member]
Costs recognized period from inception to March 31, 2019(1)	19.7
Total remaining costs expected to be incurred	0.0
Retention Expense [Member]
Costs recognized period from inception to March 31, 2019(1)	24.9
Total remaining costs expected to be incurred	15.1
Pension curtailment [Member]
Costs recognized period from inception to March 31, 2019(1)	(0.4)
Total remaining costs expected to be incurred	0.0
Total [Member]
Termination benefits	6.8	3.4
Office lease termination costs	0.6	0.0
Accelerated share-based compensation(1)	8.4	2.8
Retention expense (including share-based compensation)	6.1	1.7
Pension and Medical Plan curtailment	(0.5)	0.0
Total restructuring costs	21.4	7.9
Costs recognized period from inception to March 31, 2019(1)	84.8
Total remaining costs expected to be incurred	15.1
General and Administrative Expense [Member]
Termination benefits	6.7	3.4
Office lease termination costs	0.6	0.0
Accelerated share-based compensation(1)	6.9	2.8
Retention expense (including share-based compensation)	6.1	1.7
Pension and Medical Plan curtailment	0.0	0.0
Total restructuring costs	20.3	7.9
Net gain (loss) from asset sales [Member]
Termination benefits	0.1	0.0
Office lease termination costs	0.0	0.0
Accelerated share-based compensation(1)	1.5	0.0
Retention expense (including share-based compensation)	0.0	0.0
Pension and Medical Plan curtailment	(0.2)	0.0
Total restructuring costs	1.4	0.0
Interest and other income (expense) [Member]
Termination benefits	0.0	0.0
Office lease termination costs	0.0	0.0
Accelerated share-based compensation(1)	0.0	0.0
Retention expense (including share-based compensation)	0.0	0.0
Pension and Medical Plan curtailment	(0.3)	$ 0.0
Total incurred to date [Member]
Accelerated share-based compensation(1)	$ 8.8