Incentive Plan Incentive Plan (Schedule of Deferred and Unearned Compensation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Long-term incentive units
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Expected period to recognize unearned compensation	$ 6.8	$ 3.9	$ 4.5
Deferred compensation, capitalized	0.2	1.7	1.8
Unearned compensation	$ 11.5	6.9
Deferred compensation, expensed	2 years 10 months 20 days
Market performance-based awards
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Expected period to recognize unearned compensation	$ 12.7	9.6	7.9
Deferred compensation, capitalized	0.8	2.3	1.9
Unearned compensation	$ 24.8	24.7
Deferred compensation, expensed	2 years 6 months 1 day
Restricted stock
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Expected period to recognize unearned compensation	$ 6.1	4.5	4.1
Deferred compensation, capitalized	4.2	3.3	$ 2.8
Unearned compensation	$ 23.6	$ 17.5
Deferred compensation, expensed	2 years 8 months 20 days