Incentive Plan (Schedule of Compensation Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Long-term incentive units
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Deferred compensation, expensed	$ 8.7	$ 6.8	$ 3.9
Deferred compensation, capitalized	0.2	0.2	1.7
Unearned Compensation	$ 15.4	$ 11.5
Expected period to recognize unearned compensation (in years)	2 years 2 months 12 days	2 years 2 months 12 days
Market performance-based awards
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Deferred compensation, expensed	$ 13.0	$ 12.7	9.6
Deferred compensation, capitalized	0.8	0.8	2.3
Unearned Compensation	$ 28.4	$ 24.8
Expected period to recognize unearned compensation (in years)	2 years 6 months	2 years 6 months
Restricted stock
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Deferred compensation, expensed	$ 11.5	$ 6.1	4.5
Deferred compensation, capitalized	2.8	4.2	$ 3.3
Unearned Compensation	$ 29.1	$ 23.6
Expected period to recognize unearned compensation (in years)	2 years 7 months 6 days	2 years 7 months 6 days