Summary of Significant Accounting Policies Discussion Adoption of ASU 2017-07 (Details) - USD ($) $ in Millions	3 Months Ended									9 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating costs and expenses									$ 950.2	$ 3,045.1	$ 3,536.6	$ 4,071.4
Selling and administrative expenses									36.3	106.3	145.0	158.1
Operating profit (loss)	$ (165.5)	$ (36.8)	$ 79.5	$ 184.5	$ 126.8	$ 130.3	$ 165.3	$ 239.2	212.5	663.8	61.7	661.6
Net periodic benefit costs, excluding service cost									14.4	21.9	19.4	18.1
Net mark-to-market adjustment on actuarially determined liabilities	$ 67.4				$ 125.5				0.0	(45.2)	67.4	(125.5)
Income (loss) from continuing operations before income taxes									$ (459.3)	$ 552.1	$ (142.3)	664.1
Adjustment												$ 22.5