Incentive Plan (Schedule of Deferred and Unearned Compensation) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Long-term incentive units
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Deferred compensation, expensed	$ 1.4	$ 0.9
Deferred compensation, capitalized	0.0	$ 0.2
Unearned compensation	21.9		$ 11.5
Expected period to recognize unearned compensation (in years)		2 years 11 months 1 day
Market performance-based awards
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Deferred compensation, expensed	3.1	$ 3.1
Deferred compensation, capitalized	0.2	$ 0.3
Unearned compensation	40.7		24.8
Expected period to recognize unearned compensation (in years)		2 years 7 months 15 days
Restricted stock
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Deferred compensation, expensed	2.6	$ 1.5
Deferred compensation, capitalized	0.6	$ 1.1
Unearned compensation	$ 39.8		$ 23.6
Expected period to recognize unearned compensation (in years)		3 years 2 months 20 days