Business Segment (Summary of Information by Business Segment) (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total assets	$ 2,788.2						$ 2,788.2		$ 2,687.2
External revenues	2,839.0			$ 3,657.6			8,403.4	$ 10,574.4
Income (Loss)	66.9	$ 168.9	$ 89.3	69.2	$ 32.7	$ 5.3	325.1	107.2
Operating Segments | Marketing
Segment Reporting Information [Line Items]
Total assets	2,358.1						2,358.1
External revenues	2,839.1			3,657.6			8,403.4	10,574.3
Income (Loss)	81.7			94.0			362.2	154.7
Corporate and other assets
Segment Reporting Information [Line Items]
Total assets	430.1						430.1
External revenues	(0.1)			0.0			0.0	0.1
Income (Loss)	$ (14.8)			$ (24.8)			$ (37.1)	$ (47.5)