Derivative Instruments and Hedging Activities (Details 3) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) arising in period	$ (4.2)	$ (2.3)	$ (21.1)	$ (7.8)
Gains (losses) recorded in cost of products sold	(5.2)	(0.5)	(0.9)	12.6
Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) arising in period	(3.4)	(3.1)	(21.4)	(5.4)
Foreign Exchange Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) arising in period	(0.8)	0.8	0.3	(2.4)
Other Nonoperating Income (Expense) [Member] | Foreign Exchange Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains {losses} recognized on derivatives not designated as cash flow hedges	0.1	(0.1)	0.0	0.3
Cost of Products Sold [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recorded in cost of products sold	(5.2)	(0.5)	(0.9)	12.6
Cost of Products Sold [Member] | Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recorded in cost of products sold	(4.8)	0.1	0.4	13.1
Gains {losses} recognized on derivatives not designated as cash flow hedges	(15.3)	4.5	41.1	(1.9)
Cost of Products Sold [Member] | Foreign Exchange Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) recorded in cost of products sold	(0.4)	(0.6)	(1.3)	(0.5)
Iron Ore [Member] | Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Cash Received on Hedge	13.5	5.7	26.9	17.2
Iron Ore [Member] | Cost of Products Sold [Member] | Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains {losses} recognized on derivatives not designated as cash flow hedges	$ (15.3)	$ 4.4	$ 41.9	$ (1.2)