Income Taxes - Significant Components of our deferred tax assets & liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Deferred tax assets:
Accrued postretirement benefit cost	$ 25.7	$ 29.5
Net operating losses and tax credits	395.8	453.1
Disallowed interest expense	27.7	17.8
Derivative and hedging contracts	0.0	18.6
Fixed asset tax over book basis	17.0	0.0
Other	26.1	20.7
Total deferred tax assets	492.3	539.7
Valuation allowance	(487.9)	(485.8)
Net deferred tax assets	4.4	53.9
Deferred tax liabilities:
Fixed asset book over tax basis	(60.5)	(86.2)
Derivatives	(19.0)	0.0
Foreign basis differences	(19.8)	(19.6)
Other	(7.9)	(6.8)
Total deferred tax liabilities	(107.2)	(112.6)
Net deferred tax liability	$ (102.8)	$ (58.7)