Incentive Plan Incentive Plan (Schedule of Deferred and Unearned Compensation) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Long-term incentive units
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Deferred compensation, expensed	$ 1.2	$ 0.8	$ 5.2	$ 3.4
Deferred compensation, capitalized	0.1	0.6	0.2	1.4
Unearned compensation	11.0		$ 11.0		$ 6.9
Expected period to recognize unearned compensation (in years)			2 years 11 months 28 days
Market performance-based awards
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Deferred compensation, expensed	3.1	2.7	$ 10.0	7.3
Deferred compensation, capitalized	0.3	0.6	0.7	1.9
Unearned compensation	28.4		$ 28.4		24.7
Expected period to recognize unearned compensation (in years)			2 years 1 month 2 days
Restricted stock
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Deferred compensation, expensed	1.5	1.1	$ 4.7	3.3
Deferred compensation, capitalized	1.1	$ 0.8	3.2	$ 2.5
Unearned compensation	$ 23.4		$ 23.4		$ 17.5
Expected period to recognize unearned compensation (in years)			2 years 9 months