Business Segment (Summary of Information by Business Segment) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total assets	$ 2,860.7				$ 2,860.7		$ 2,687.2
External revenues	2,379.6		$ 3,800.4		5,564.4	$ 6,916.8
Income (Loss)	168.9	$ 89.3	32.7	$ 5.3	258.2	38.0
Operating Segments | Marketing
Segment Reporting Information [Line Items]
Total assets	2,353.2				2,353.2
External revenues	2,379.6		3,800.4		5,564.3	6,916.7
Income (Loss)	179.6		44.5		280.5	60.7
Corporate and other assets
Segment Reporting Information [Line Items]
Total assets	507.5				507.5
External revenues	0.0		0.0		0.1	0.1
Income (Loss)	$ (10.7)		$ (11.8)		$ (22.3)	$ (22.7)