Consolidated Balance Sheets - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2013
Current assets
Cash and cash equivalents	$ 200.3	$ 280.3
Accounts receivable—trade, less allowance for doubtful accounts of $.1 in 2020 and $1.2 in 2019	136.3	172.9
Inventories, at lower of cost or market	243.5	227.6
Prepaid expenses and other current assets	22.5	30.0
Total current assets	602.6	710.8
Property, plant and equipment, at cost less accumulated depreciation and amortization of $1,107.3 in 2020 and $1,079.2 in 2019	1,808.3	1,807.3
Other assets	172.5	169.1
Total assets	2,583.4	2,687.2
Current liabilities
Current maturities of long-term debt	51.2	38.8
Trade accounts payable and accrued liabilities	400.6	466.2
Income taxes payable	6.1	0.0
Total current liabilities	457.9	505.0
Long-term debt, including capitalized lease obligations	987.4	999.3
Deferred income taxes	221.8	216.7
Asset retirement obligations	33.2	32.8
Deferred credits and other liabilities	136.3	130.4
Total liabilities	1,836.6	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 2019 and 2018, respectively)	0.5	0.5
Treasury stock (17,589,526 and 16,307,048 shares held at 2020 and 2019, respectively)	(1,235.2)	(1,099.8)
Additional paid in capital (APIC)	530.2	538.7
Retained earnings	1,453.3	1,362.9
Accumulated other comprehensive income (loss) (AOCI)	(2.0)	0.7
Total stockholders' equity	746.8	803.0
Total liabilities and stockholders' equity	$ 2,583.4	$ 2,687.2
Preferred stock shares authorized (in shares)	20,000,000	20,000,000
Preferred stock shares outstanding (in shares)	0	0
Common stock shares authorized (in shares)	200,000,000	200,000,000
Common stock shares issued (in shares)	46,767,164	46,767,164
Preferred stock par value (in dollars per share)	$ 0.01	$ 0.01
Common stock par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01