Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 317.5			$ 280.3
Accounts receivable—trade, less allowance for doubtful accounts of $0.1 at 2020 and $1.2 at 2019	142.7			172.9
Inventories, at lower of cost or market	268.1			227.6
Prepaid expenses and other current assets	14.9			30.0
Total current assets	743.2			710.8
Property, plant and equipment, at cost less accumulated depreciation and amortization of $1,151.3 at 2020 and $1,079.2 at 2019	1,854.0			1,807.3
Other assets	191.0			169.1
Total assets	2,788.2			2,687.2
Current liabilities
Current maturities of long-term debt	51.2			38.8
Trade accounts payable and accrued liabilities	442.8			466.2
Income taxes payable	19.4			0.0
Total current liabilities	513.4			505.0
Long-term debt, including capitalized lease obligations	963.2			999.3
Deferred income taxes	217.5			216.7
Asset retirement obligations	34.0			32.8
Deferred credits and other liabilities	164.4			130.4
Total liabilities	1,892.5			1,884.2
Stockholders' Equity
Preferred Stock, par $0.01, (authorized 20,000,000 shares, none outstanding)	0.0			0.0
Common Stock, par $0.01, (authorized 200,000,000 shares, 46,767,164 shares issued at 2020 and 2019, respectively)	0.5			0.5
Treasury stock (18,199,448 and 16,307,048 shares held at 2020 and 2019, respectively)	(1,321.8)			(1,099.8)
Additional paid in capital (APIC)	530.0			538.7
Retained earnings	1,689.1			1,362.9
Accumulated other comprehensive income (loss) (AOCI)	(2.1)			0.7
Total stockholders' equity	895.7	$ 918.0	$ 746.8	803.0	$ 779.0	$ 816.3	$ 797.8	$ 807.3
Total liabilities and stockholders' equity	$ 2,788.2			$ 2,687.2