Guarantor Subsidiaries (Consolidating Balance Sheet) (Details) - 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	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	1,807,300	1,748,200
Investments in subsidiaries	0	0
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
Inter-company accounts payable	0	0
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	0	0
Common Stock, par $0.01	500	500
Treasury stock	(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
Eliminations
Current assets
Cash and cash equivalents	0	0	0	0
Accounts receivable—trade, less allowance for doubtful accounts	0	0
Inventories, at lower of cost or market	0	0
Prepaid expenses and other current assets	0	0
Total current assets	0	0
Property, plant and equipment, at cost less accumulated depreciation and amortization of $1,079.2 in 2019 and $974.2 in 2018	0
Property, plant and equipment, at cost less accumulated depreciation and amortization		0
Investments in subsidiaries	(2,735,700)	(2,581,400)
Other assets	0	0
Total assets	(2,735,700)	(2,581,400)
Current liabilities
Current maturities of long-term debt	0	0
Inter-company accounts payable	0	0
Trade accounts payable and accrued liabilities	0	0
Total current liabilities	0	0
Long-term debt, including capitalized lease obligations	0	0
Deferred income taxes	0	0
Asset retirement obligations	0	0
Deferred credits and other liabilities	0	0
Total liabilities	0	0
Stockholders' Equity
Preferred Stock, par $0.01	0	0
Common Stock, par $0.01	(100)	(100)
Treasury stock	0	0
Additional paid in capital (APIC)	(1,368,400)	(1,368,400)
Retained earnings	(1,367,200)	(1,212,900)
Accumulated other comprehensive income (AOCI)	0	0
Total stockholders' equity	(2,735,700)	(2,581,400)
Total liabilities and stockholders' equity	(2,735,700)	(2,581,400)
Parent Company | Reportable Legal Entities
Current assets
Cash and cash equivalents	0	0	0	0
Accounts receivable—trade, less allowance for doubtful accounts	0	0
Inventories, at lower of cost or market	0	0
Prepaid expenses and other current assets	0	0
Total current assets	0	0
Property, plant and equipment, at cost less accumulated depreciation and amortization of $1,079.2 in 2019 and $974.2 in 2018	0
Property, plant and equipment, at cost less accumulated depreciation and amortization		0
Investments in subsidiaries	2,591,800	2,437,000
Other assets	0	0
Total assets	2,591,800	2,437,000
Current liabilities
Current maturities of long-term debt	0	0
Inter-company accounts payable	(100)	(100)
Trade accounts payable and accrued liabilities	0	0
Total current liabilities	(100)	(100)
Long-term debt, including capitalized lease obligations	0	0
Deferred income taxes	0	0
Asset retirement obligations	0	0
Deferred credits and other liabilities	0	0
Total liabilities	(100)	(100)
Stockholders' Equity
Preferred Stock, par $0.01	0	0
Common Stock, par $0.01	500	500
Treasury stock	(1,099,800)	(940,300)
Additional paid in capital (APIC)	1,188,800	1,195,100
Retained earnings	2,502,400	2,181,800
Accumulated other comprehensive income (AOCI)	0	0
Total stockholders' equity	2,591,900	2,437,100
Total liabilities and stockholders' equity	2,591,800	2,437,000
Issuer | Reportable Legal Entities
Current assets
Cash and cash equivalents	279,400	184,000	169,900	153,800
Accounts receivable—trade, less allowance for doubtful accounts	173,000	138,800
Inventories, at lower of cost or market	227,600	221,500
Prepaid expenses and other current assets	29,600	25,100
Total current assets	709,600	569,400
Property, plant and equipment, at cost less accumulated depreciation and amortization of $1,079.2 in 2019 and $974.2 in 2018	1,799,100
Property, plant and equipment, at cost less accumulated depreciation and amortization		1,745,900
Investments in subsidiaries	143,900	144,400
Other assets	169,100	42,500
Total assets	2,821,700	2,502,200
Current liabilities
Current maturities of long-term debt	38,800	21,200
Inter-company accounts payable	196,100	203,000
Trade accounts payable and accrued liabilities	466,200	456,900
Total current liabilities	701,100	681,100
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	2,080,300	1,756,500
Stockholders' Equity
Preferred Stock, par $0.01	0	0
Common Stock, par $0.01	0	0
Treasury stock	0	0
Additional paid in capital (APIC)	578,800	572,800
Retained earnings	161,900	172,900
Accumulated other comprehensive income (AOCI)	700	0
Total stockholders' equity	741,400	745,700
Total liabilities and stockholders' equity	2,821,700	2,502,200
Guarantor Subsidiaries | Reportable Legal Entities
Current assets
Cash and cash equivalents	900	500	100	0
Accounts receivable—trade, less allowance for doubtful accounts	(100)	0
Inventories, at lower of cost or market	0	0
Prepaid expenses and other current assets	400	200
Total current assets	1,200	700
Property, plant and equipment, at cost less accumulated depreciation and amortization of $1,079.2 in 2019 and $974.2 in 2018	8,200
Property, plant and equipment, at cost less accumulated depreciation and amortization		2,300
Investments in subsidiaries	0	0
Other assets	0	0
Total assets	9,400	3,000
Current liabilities
Current maturities of long-term debt	0	0
Inter-company accounts payable	(41,700)	(48,600)
Trade accounts payable and accrued liabilities	0	0
Total current liabilities	(41,700)	(48,600)
Long-term debt, including capitalized lease obligations	0	0
Deferred income taxes	0	0
Asset retirement obligations	0	0
Deferred credits and other liabilities	0	0
Total liabilities	(41,700)	(48,600)
Stockholders' Equity
Preferred Stock, par $0.01	0	0
Common Stock, par $0.01	100	100
Treasury stock	0	0
Additional paid in capital (APIC)	52,000	52,000
Retained earnings	(1,000)	(500)
Accumulated other comprehensive income (AOCI)	0	0
Total stockholders' equity	51,100	51,600
Total liabilities and stockholders' equity	9,400	3,000
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets
Cash and cash equivalents	0	0	$ 0	$ 0
Accounts receivable—trade, less allowance for doubtful accounts	0	0
Inventories, at lower of cost or market	0	0
Prepaid expenses and other current assets	0	0
Total current assets	0	0
Property, plant and equipment, at cost less accumulated depreciation and amortization of $1,079.2 in 2019 and $974.2 in 2018	0
Property, plant and equipment, at cost less accumulated depreciation and amortization		0
Investments in subsidiaries	0	0
Other assets	0	0
Total assets	0	0
Current liabilities
Current maturities of long-term debt	0	0
Inter-company accounts payable	(154,300)	(154,300)
Trade accounts payable and accrued liabilities	0	0
Total current liabilities	(154,300)	(154,300)
Long-term debt, including capitalized lease obligations	0	0
Deferred income taxes	0	0
Asset retirement obligations	0	0
Deferred credits and other liabilities	0	0
Total liabilities	(154,300)	(154,300)
Stockholders' Equity
Preferred Stock, par $0.01	0	0
Common Stock, par $0.01	0	0
Treasury stock	0	0
Additional paid in capital (APIC)	87,500	87,500
Retained earnings	66,800	66,800
Accumulated other comprehensive income (AOCI)	0	0
Total stockholders' equity	154,300	154,300
Total liabilities and stockholders' equity	$ 0	$ 0