Income Taxes (Deferred tax liabilities and assets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Depreciation	$ 148.0	$ 229.5
Deferred revenue	14.4	15.8
Intangibles	6.4	8.9
Gain on debt redemption	7.9	25.7
State taxes	22.1	30.2
Other	10.6	31.5
Deferred tax liabilities	209.4	341.6
Claims and insurance	(98.2)	(149.6)
Net operating loss carryforwards	(228.0)	(302.9)
Employee benefit accruals	(88.8)	(201.8)
Deferred Tax Asset, Sale Leaseback Transaction	(64.7)	(99.9)
Other	(31.7)	(46.0)
Deferred tax assets	(511.4)	(800.2)
Valuation allowance	305.1	461.7
Net deferred tax assets	(206.3)	(338.5)
Net deferred tax liability	$ 3.1	$ 3.1