Income Taxes Income Taxes - Summary of Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax assets:
Federal net operating loss carryforward	$ 54.5	$ 59.5
State net operating loss carryforward	14.2	6.5
Asset retirement obligations and other	0.0	0.9
Total deferred tax assets	68.7	66.9
Deferred tax liabilities:
Property, equipment, and intangible assets	(414.9)	(609.5)
Deferred tax liability, net	$ (346.2)	$ (542.6)