Consolidated Balance Sheets - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2013
Current assets
Cash and cash equivalents	$ 403.6		$ 280.3
Accounts receivable—trade, less allowance for doubtful accounts	161.3		172.9
Inventories, at lower of cost or market	257.7		227.6
Prepaid expenses and other current assets	17.7		30.0
Total current assets	840.3		710.8
Property, Plant and Equipment, Net	1,834.3		1,807.3
Other assets	186.1		169.1
Total assets	2,860.7		2,687.2
Current liabilities
Current maturities of long-term debt	51.2		38.8
Trade accounts payable and accrued liabilities	461.7		466.2
Income taxes payable	40.8		0.0
Total current liabilities	553.7		505.0
Long-term debt, including capitalized lease obligations	975.3		999.3
Deferred income taxes	225.2		216.7
Asset retirement obligations	33.7		32.8
Deferred credits and other liabilities	154.8		130.4
Total liabilities	1,942.7		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)	533.0		538.7
Retained earnings	1,622.2		1,362.9
Accumulated other comprehensive income (loss) (AOCI)	(2.5)		0.7
Total stockholders' equity	918.0	$ 746.8	803.0	$ 816.3	$ 797.8	$ 807.3
Total liabilities and stockholders' equity	$ 2,860.7		$ 2,687.2
Preferred stock par value (in dollars per share)	$ 0.01		$ 0.01
Preferred stock shares authorized (in shares)	20,000,000		20,000,000
Preferred stock shares outstanding (in shares)	0		0
Common stock par value (in dollars per share)	$ 0.01		$ 0.01				$ 0.01
Common stock shares authorized (in shares)	200,000,000		200,000,000
Common stock shares issued (in shares)	46,767,164		46,767,164