Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Cash Flows from Operating Activities
Net earnings including noncontrolling interest	$ 249.7	$ 229.6	$ 357.9	$ 325.6
Adjustments to reconcile net earnings including noncontrolling interests to net cash provided by operating activities:
Depreciation, depletion and amortization	214.5	168.8	648.8	493.5
Business Combination, Inventory Acquired, Amortization of Fair Value Adjustment			(47.4)	0.0
Deferred and other income taxes			(12.3)	53.9
Equity in net loss (earnings) of nonconsolidated companies, net of dividends			11.0	(15.5)
Accretion expense for asset retirement obligations			37.2	19.6	$ 25.7
Share-based compensation expense			25.7	24.5
Unrealized loss (gain) on derivatives			14.3	(5.7)
Other			21.2	(38.3)
Changes in assets and liabilities, excluding effects of acquisition:
Receivables, net			(31.6)	(6.8)
Inventories			(198.1)	(254.6)
Other current and noncurrent assets			(75.9)	(42.7)
Accounts payable and accrued liabilities			466.5	(38.4)
Other noncurrent liabilities			42.5	9.2
Net cash provided by operating activities			1,259.8	524.3
Cash Flows from Investing Activities
Capital expenditures	(241.0)	(197.6)	(665.4)	(589.9)
Purchases of available-for-sale securities - restricted			(486.1)	(1,546.3)
Proceeds from sale of available-for-sale securities - restricted			470.5	1,533.7
Payments to Acquire Equity Method Investments			0.0	(62.5)
Investments in consolidated affiliate			(3.6)	(47.7)
Proceeds from Sale of Property, Plant, and Equipment			9.3	69.1
Payments to Acquire Businesses, Gross			(985.3)	0.0
Other			(0.3)	0.3
Net cash used in investing activities			(1,660.9)	(643.3)
Cash Flows from Financing Activities
Payments of short-term debt			(120.1)	(523.2)
Proceeds from issuance of short-term debt			145.2	608.1
Payments of structured accounts payable arrangements			(582.4)	(238.8)
Proceeds from structured accounts payable arrangements			590.2	473.8
Payments of long-term debt			(722.4)	(6.2)
Proceeds from issuance of long-term debt			39.3	1.5
Cash dividends paid			(28.9)	(201.8)
Other			(0.5)	(2.2)
Net cash (used in) provided by financing activities			(679.6)	111.2
Effect of exchange rate changes on cash			(62.8)	22.8
Net change in cash, cash equivalents and restricted cash			(1,143.5)	15.0
Cash, cash equivalents and restricted cash-beginning of period			2,194.4	711.4	711.4
Cash, cash equivalents and restricted cash-end of period	1,050.9	726.4	1,050.9	726.4	2,194.4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents						$ 1,029.9	$ 2,153.5	$ 685.7
Restricted cash in other current assets						8.2		7.5
Restricted cash in other assets						12.8	32.6	33.2
Cash, cash equivalents and restricted cash-end of period	$ 1,050.9	$ 726.4	2,194.4	711.4	$ 711.4	$ 1,050.9	$ 2,194.4	$ 726.4
Supplemental Disclosure of Cash Flow Information:
Interest (net of amount capitalized)			97.6	98.4
Income taxes (net of refunds)			$ 13.2	$ (16.7)