Restructuring Costs Restructuring Expense Table (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Termination Benefits [Member]
Costs recognized from inception to September 30, 2019(1)	$ 43.2		$ 43.2
Total remaining costs expected to be incurred			0.0
Office lease Termination Costs [Member]
Costs recognized from inception to September 30, 2019(1)	1.6		1.6
Total remaining costs expected to be incurred			0.0
Accelerated Share Based Compensation [Member]
Costs recognized from inception to September 30, 2019(1)	22.6		22.6
Total remaining costs expected to be incurred			0.0
Retention Expense [Member]
Costs recognized from inception to September 30, 2019(1)	34.2		34.2
Total remaining costs expected to be incurred			5.0
Pension curtailment [Member]
Costs recognized from inception to September 30, 2019(1)	(0.2)		(0.2)
Total remaining costs expected to be incurred			0.0
Total [Member]
Termination benefits	4.3	$ 6.7	11.0	$ 13.7
Office lease termination costs		0.7	0.6	1.0
Accelerated share-based compensation(1)	1.6	3.2	11.3	7.2
Retention expense (including share-based compensation)	4.5	5.8	15.4	13.8
Pension and Medical Plan curtailment		0.3	(0.4)	0.3
Total restructuring costs	10.4	16.7	37.9	36.0
Costs recognized from inception to September 30, 2019(1)	101.4		101.4
Total remaining costs expected to be incurred			5.0
General and Administrative Expense [Member]
Termination benefits	4.3	5.3	10.9	10.4
Office lease termination costs		0.7	0.6	1.0
Accelerated share-based compensation(1)	1.6	1.0	9.8	5.0
Retention expense (including share-based compensation)	4.5	5.8	15.4	13.8
Pension and Medical Plan curtailment		0.0	0.0	0.0
Total restructuring costs	10.4	12.8	36.7	30.2
Net gain (loss) from asset sales [Member]
Termination benefits	0.0	1.4	0.1	3.3
Office lease termination costs		0.0	0.0	0.0
Accelerated share-based compensation(1)	0.0	2.2	1.5	2.2
Retention expense (including share-based compensation)	0.0	0.0	0.0	0.0
Pension and Medical Plan curtailment		0.0	(0.2)	0.0
Total restructuring costs	0.0	3.6	1.4	5.5
Interest and other income (expense) [Member]
Termination benefits	0.0	0.0	0.0	0.0
Office lease termination costs		0.0	0.0	0.0
Accelerated share-based compensation(1)	0.0	0.0	0.0	0.0
Retention expense (including share-based compensation)	0.0	0.0	0.0	0.0
Pension and Medical Plan curtailment		0.3	(0.2)	0.3
Total restructuring costs	0.0	$ 0.3	(0.2)	$ 0.3
Net gain (loss) from asset sales [Member] | Total [Member]
Pension and Medical Plan curtailment			(0.5)
Total incurred to date [Member]
Accelerated share-based compensation(1)	$ 3.5		$ 25.2