Incentive Plan Incentive Plan (Schedule of Deferred and Unearned Compensation) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Long-term incentive units
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Deferred compensation, expensed	$ 2.9	$ 1.9	$ 3.8	$ 2.6
Deferred compensation, capitalized	(0.1)	0.4	0.1	0.8
Unearned compensation	13.2		$ 13.2		$ 6.9
Expected period to recognize unearned compensation (in years)			3 years 10 days
Market performance-based awards
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Deferred compensation, expensed	3.5	2.6	$ 6.6	4.6
Deferred compensation, capitalized	0.4	0.5	0.7	1.2
Unearned compensation	34.8		$ 34.8		24.7
Expected period to recognize unearned compensation (in years)			2 years 6 months 2 days
Restricted stock
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Deferred compensation, expensed	1.7	1.1	$ 3.2	2.1
Deferred compensation, capitalized	1.1	$ 0.8	2.1	$ 1.7
Unearned compensation	$ 26.5		$ 26.5		$ 17.5
Expected period to recognize unearned compensation (in years)			2 years 11 months 7 days