Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 280,300	$ 184,500	$ 170,000	$ 153,800
Accounts receivable—trade, less allowance for doubtful accounts of $1.2 in 2019 and $1.1 in 2018	172,900	138,800
Inventories, at lower of cost or market	227,600	221,500
Prepaid expenses and other current assets	30,000	25,300
Total current assets	710,800	570,100
Property, plant and equipment, at cost less accumulated depreciation and amortization of $1,079.2 in 2019 and $974.2 in 2018	1,807,300
Property, plant and equipment, at cost less accumulated depreciation and amortization of $1,079.2 in 2019 and $974.2 in 2018	1,807,300	1,748,200
Other assets	169,100	42,500
Total assets	2,687,200	2,360,800	2,331,000
Current liabilities
Current maturities of long-term debt	38,800	21,200
Trade accounts payable and accrued liabilities	466,200	456,900
Total current liabilities	505,000	478,100
Long-term debt, including capitalized lease obligations	999,300	842,100
Deferred income taxes	216,700	192,200
Asset retirement obligations	32,800	30,700
Deferred credits and other liabilities	130,400	10,400
Total liabilities	1,884,200	1,553,500
Stockholders' Equity
Preferred Stock, par $0.01 (authorized 20,000,000 shares, none outstanding)	0	0
Common Stock, par $0.01, (authorized 200,000,000 shares, 46,767,164 shares issued at December 2019 and 2018, respectively)	500	500
Treasury stock (16,307,048 and 14,505,681 shares held at December 31, 2019 and 2018, respectively)	(1,099,800)	(940,300)
Additional paid in capital (APIC)	538,700	539,000
Retained earnings	1,362,900	1,208,100
Accumulated other comprehensive income (AOCI)	700	0
Total stockholders' equity	803,000	807,300	$ 738,400	$ 697,000
Total liabilities and stockholders' equity	$ 2,687,200	$ 2,360,800