Guarantor Subsidiaries (Consolidating Balance Sheet) (Details) - USD ($) $ / shares in Units, $ 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	Sep. 30, 2013
Current assets
Cash and cash equivalents	$ 317.5			$ 280.3
Accounts receivable—trade, less allowance for doubtful accounts	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	1,854.0			1,807.3
Investments in subsidiaries	0.0			0.0
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
Inter-company accounts payable	0.0			0.0
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	0.0			0.0
Common Stock	0.5			0.5
Treasury Stock	(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
Allowance for doubtful accounts	0.1			1.2
Property, plant and equipment, accumulated depreciation and amortization	$ 1,151.3			$ 1,079.2
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
Treasury stock, shares held (in shares)	18,199,448			16,307,048
Eliminations
Current assets
Cash and cash equivalents	$ 0.0			$ 0.0
Accounts receivable—trade, less allowance for doubtful accounts	0.0			0.0
Inventories, at lower of cost or market	0.0			0.0
Prepaid expenses and other current assets	0.0			0.0
Total current assets	0.0			0.0
Property, plant and equipment, at cost less accumulated depreciation and amortization	0.0			0.0
Investments in subsidiaries	(3,060.3)			(2,735.7)
Other assets	0.0			0.0
Total assets	(3,060.3)			(2,735.7)
Current liabilities
Current maturities of long-term debt	0.0			0.0
Inter-company accounts payable	0.0			0.0
Trade accounts payable and accrued liabilities	0.0			0.0
Income taxes payable	0.0
Total current liabilities	0.0			0.0
Long-term debt, including capitalized lease obligations	0.0			0.0
Deferred income taxes	0.0			0.0
Asset retirement obligations	0.0			0.0
Deferred credits and other liabilities	0.0			0.0
Total liabilities	0.0			0.0
Stockholders' Equity
Preferred Stock	0.0			0.0
Common Stock	(0.1)			(0.1)
Treasury Stock	0.0			0.0
Additional paid in capital (APIC)	(1,368.4)			(1,368.4)
Retained earnings	(1,691.8)			(1,367.2)
Accumulated other comprehensive income (loss) (AOCI)	0.0			0.0
Total stockholders' equity	(3,060.3)			(2,735.7)
Total liabilities and stockholders' equity	(3,060.3)			(2,735.7)
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			0.0
Inventories, at lower of cost or market	0.0			0.0
Prepaid expenses and other current assets	0.0			0.0
Total current assets	0.0			0.0
Property, plant and equipment, at cost less accumulated depreciation and amortization	0.0			0.0
Investments in subsidiaries	2,916.9			2,591.8
Other assets	0.0			0.0
Total assets	2,916.9			2,591.8
Current liabilities
Current maturities of long-term debt	0.0			0.0
Inter-company accounts payable	230.9			(0.1)
Trade accounts payable and accrued liabilities	0.0			0.0
Income taxes payable	0.0
Total current liabilities	230.9			(0.1)
Long-term debt, including capitalized lease obligations	0.0			0.0
Deferred income taxes	0.0			0.0
Asset retirement obligations	0.0			0.0
Deferred credits and other liabilities	0.0			0.0
Total liabilities	230.9			(0.1)
Stockholders' Equity
Preferred Stock	0.0			0.0
Common Stock	0.5			0.5
Treasury Stock	(1,321.8)			(1,099.8)
Additional paid in capital (APIC)	1,179.8			1,188.8
Retained earnings	2,827.5			2,502.4
Accumulated other comprehensive income (loss) (AOCI)	0.0			0.0
Total stockholders' equity	2,686.0			2,591.9
Total liabilities and stockholders' equity	2,916.9			2,591.8
Issuer | Reportable Legal Entities
Current assets
Cash and cash equivalents	315.8			279.4
Accounts receivable—trade, less allowance for doubtful accounts	142.6			173.0
Inventories, at lower of cost or market	268.1			227.6
Prepaid expenses and other current assets	14.8			29.6
Total current assets	741.3			709.6
Property, plant and equipment, at cost less accumulated depreciation and amortization	1,842.8			1,799.1
Investments in subsidiaries	143.4			143.9
Other assets	191.0			169.1
Total assets	2,918.5			2,821.7
Current liabilities
Current maturities of long-term debt	51.2			38.8
Inter-company accounts payable	(39.2)			196.1
Trade accounts payable and accrued liabilities	442.5			466.2
Income taxes payable	19.8
Total current liabilities	474.3			701.1
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	1,853.4			2,080.3
Stockholders' Equity
Preferred Stock	0.0			0.0
Common Stock	0.0			0.0
Treasury Stock	0.0			0.0
Additional paid in capital (APIC)	579.1			578.8
Retained earnings	488.1			161.9
Accumulated other comprehensive income (loss) (AOCI)	(2.1)			0.7
Total stockholders' equity	1,065.1			741.4
Total liabilities and stockholders' equity	2,918.5			2,821.7
Guarantor Subsidiaries | Reportable Legal Entities
Current assets
Cash and cash equivalents	1.7			0.9
Accounts receivable—trade, less allowance for doubtful accounts	0.1			(0.1)
Inventories, at lower of cost or market	0.0			0.0
Prepaid expenses and other current assets	0.1			0.4
Total current assets	1.9			1.2
Property, plant and equipment, at cost less accumulated depreciation and amortization	11.2			8.2
Investments in subsidiaries	0.0			0.0
Other assets	0.0			0.0
Total assets	13.1			9.4
Current liabilities
Current maturities of long-term debt	0.0			0.0
Inter-company accounts payable	(37.4)			(41.7)
Trade accounts payable and accrued liabilities	0.3			0.0
Income taxes payable	(0.4)
Total current liabilities	(37.5)			(41.7)
Long-term debt, including capitalized lease obligations	0.0			0.0
Deferred income taxes	0.0			0.0
Asset retirement obligations	0.0			0.0
Deferred credits and other liabilities	0.0			0.0
Total liabilities	(37.5)			(41.7)
Stockholders' Equity
Preferred Stock	0.0			0.0
Common Stock	0.1			0.1
Treasury Stock	0.0			0.0
Additional paid in capital (APIC)	52.0			52.0
Retained earnings	(1.5)			(1.0)
Accumulated other comprehensive income (loss) (AOCI)	0.0			0.0
Total stockholders' equity	50.6			51.1
Total liabilities and stockholders' equity	13.1			9.4
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			0.0
Inventories, at lower of cost or market	0.0			0.0
Prepaid expenses and other current assets	0.0			0.0
Total current assets	0.0			0.0
Property, plant and equipment, at cost less accumulated depreciation and amortization	0.0			0.0
Investments in subsidiaries	0.0			0.0
Other assets	0.0			0.0
Total assets	0.0			0.0
Current liabilities
Current maturities of long-term debt	0.0			0.0
Inter-company accounts payable	(154.3)			(154.3)
Trade accounts payable and accrued liabilities	0.0			0.0
Income taxes payable	0.0
Total current liabilities	(154.3)			(154.3)
Long-term debt, including capitalized lease obligations	0.0			0.0
Deferred income taxes	0.0			0.0
Asset retirement obligations	0.0			0.0
Deferred credits and other liabilities	0.0			0.0
Total liabilities	(154.3)			(154.3)
Stockholders' Equity
Preferred Stock	0.0			0.0
Common Stock	0.0			0.0
Treasury Stock	0.0			0.0
Additional paid in capital (APIC)	87.5			87.5
Retained earnings	66.8			66.8
Accumulated other comprehensive income (loss) (AOCI)	0.0			0.0
Total stockholders' equity	154.3			154.3
Total liabilities and stockholders' equity	$ 0.0			$ 0.0