Changes in Net Unrealized Gains (Losses) of Level 3 Net Financial Assets (Details) - Coal Trading [Member] - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
		Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Predecessor
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	[1]	$ 0.3		$ 0.0	$ (6.2)
Successor
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	[1]		$ 0.0

[1] Within the consolidated statements of operations and consolidated statements of comprehensive income for the periods presented, unrealized gains and losses from Level 3 items are combined with unrealized gains and losses on positions classified in Level 1 or 2, as well as other positions that have been realized during the applicable periods.