Income Taxes - Schedule of Deferred Income Taxes (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets
Net operating loss carryforwards	$ 65.1	$ 58.9
Unfunded pension liability and other postretirement obligations	0.5	5.6
Tax credit carryforwards	10.8	13.0
Stock based compensation	7.1	16.2
Other comprehensive income	0.2	4.6
Reserves and other	13.5	20.8
Subtotal	97.2	119.1
Valuation allowance	(54.9)	(58.9)
Total assets	42.3	60.2
Liabilities
Accelerated depreciation	(144.1)	(160.6)
Accelerated amortization	(11.3)	(12.9)
Other	(1.3)	(0.4)
Total liabilities	(156.7)	(173.9)
Net deferred tax liabilities	$ (114.4)	$ (113.7)