Derivative Financial Instruments and Related Hedging Programs, Level 3 Fair Value Input Reconciliation Table (Details) - Derivative - Midwest premium swap contracts - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of activity for financial instruments classified as Level 3:
Fair value measurement at beginning of period	$ 0.6	$ 0.6	$ 1.0
Total realized/unrealized (loss) gain included in:
Cost of goods sold, excluding depreciation and amortization and other items and Unrealized loss (gain) on derivative instruments	0.6		(3.9)
Transactions involving Level 3 derivative contracts:
Purchases	(1.2)		(4.0)
Sales	0.0		0.0
Issuances	0.0		0.0
Settlements	0.4		7.5
Transactions involving Level 3 derivatives - net	(0.8)		3.5
Transfers in and (or) out of Level 3 valuation hierarchy	$ 0.8		0.0
Fair value measurement at end of period		0.0	0.6
Total loss included in Unrealized loss (gain) on derivative instruments, attributable to the change in unrealized gain/loss relating to derivative contracts held at December 31:		$ 0.0	$ 0.6