Fair Value Measurements, Level 3 Fair Value Input Reconciliation Table (Details) - Derivative - Midwest premium swap contracts $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($)
Reconciliation of activity for financial instruments classified as Level 3:
Balance at December 31, 2014	$ 1.0
Total realized/unrealized losses included in:
Cost of goods sold excluding depreciation and amortization and other items and Unrealized losses on derivative instruments	(3.9)
Transactions involving Level 3 derivative contracts:
Purchases	(5.4)
Sales	0.0
Issuances	0.0
Settlements	6.4
Transactions involving Level 3 derivatives — net	1.0
Transfers in and/or out of Level 3 valuation hierarchy	0.0
Balance at September 30, 2015	(1.9)
Total losses included in Unrealized losses on derivative instruments, attributable to the change in unrealized gains/losses relating to derivative contracts held at September 30, 2015:	$ (2.0)