Income Taxes Schedule of Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Deferred tax liabilities
Property, plant and equipment	$ 592.9	$ 665.1
Commodity price derivatives	0.0	30.1
Deferred Tax Liabilities Operating Lease Right Of Use Assets	12.7	0.0
Other	0.9	2.6
Deferred Tax Liabilities, Gross	606.5	697.8
Deferred tax assets
NOL and tax credit carryforwards	337.7	467.9
Deferred Tax Assets State NOL Valuation Allowance	(98.8)	(82.3)
Employee benefits and compensation costs	22.3	33.2
Interest carryforward(1)	45.7	0.0
Commodity price derivatives	3.9	0.0
Deferred Tax Assets Operating Lease Liabilities	14.1	0.0
Other	7.1	9.8
Total deferred tax assets	332.0	428.6
Net deferred income tax liability	274.5	269.2
Balance sheet classification
Deferred income tax liability – noncurrent	274.5	269.2
Net deferred income tax liability	$ 274.5	$ 269.2