Guarantor Subsidiaries (Consolidating Balance Sheet) (Details) - USD ($) $ / shares in Units, $ 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	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	1,808.3	1,807.3
Investments in subsidiaries	0.0	0.0
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
Inter-company accounts payable	0.0	0.0
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	0.0	0.0
Common Stock	0.5	0.5
Treasury Stock	(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
Allowance for doubtful accounts	0.1	1.2
Property, plant and equipment, accumulated depreciation and amortization	$ 1,107.3	$ 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)	17,589,526	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	(2,824.9)	(2,735.7)
Other assets	0.0	0.0
Total assets	(2,824.9)	(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	(1,456.4)	(1,367.2)
Accumulated other comprehensive income (loss) (AOCI)	0.0	0.0
Total stockholders' equity	(2,824.9)	(2,735.7)
Total liabilities and stockholders' equity	(2,824.9)	(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	2,681.1	2,591.8
Other assets	0.0	0.0
Total assets	2,681.1	2,591.8
Current liabilities
Current maturities of long-term debt	0.0	0.0
Inter-company accounts payable	141.0	(0.1)
Trade accounts payable and accrued liabilities	0.0	0.0
Income taxes payable	0.0
Total current liabilities	141.0	(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	141.0	(0.1)
Stockholders' Equity
Preferred Stock	0.0	0.0
Common Stock	0.5	0.5
Treasury Stock	(1,235.2)	(1,099.8)
Additional paid in capital (APIC)	1,183.1	1,188.8
Retained earnings	2,591.7	2,502.4
Accumulated other comprehensive income (loss) (AOCI)	0.0	0.0
Total stockholders' equity	2,540.1	2,591.9
Total liabilities and stockholders' equity	2,681.1	2,591.8
Issuer | Reportable Legal Entities
Current assets
Cash and cash equivalents	199.0	279.4
Accounts receivable—trade, less allowance for doubtful accounts	136.4	173.0
Inventories, at lower of cost or market	243.5	227.6
Prepaid expenses and other current assets	22.5	29.6
Total current assets	601.4	709.6
Property, plant and equipment, at cost less accumulated depreciation and amortization	1,799.3	1,799.1
Investments in subsidiaries	143.8	143.9
Other assets	172.5	169.1
Total assets	2,717.0	2,821.7
Current liabilities
Current maturities of long-term debt	51.2	38.8
Inter-company accounts payable	53.6	196.1
Trade accounts payable and accrued liabilities	400.7	466.2
Income taxes payable	6.5
Total current liabilities	512.0	701.1
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	1,890.7	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)	576.0	578.8
Retained earnings	252.3	161.9
Accumulated other comprehensive income (loss) (AOCI)	(2.0)	0.7
Total stockholders' equity	826.3	741.4
Total liabilities and stockholders' equity	2,717.0	2,821.7
Guarantor Subsidiaries | Reportable Legal Entities
Current assets
Cash and cash equivalents	1.3	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	0.0	0.4
Total current assets	1.2	1.2
Property, plant and equipment, at cost less accumulated depreciation and amortization	9.0	8.2
Investments in subsidiaries	0.0	0.0
Other assets	0.0	0.0
Total assets	10.2	9.4
Current liabilities
Current maturities of long-term debt	0.0	0.0
Inter-company accounts payable	(40.3)	(41.7)
Trade accounts payable and accrued liabilities	(0.1)	0.0
Income taxes payable	(0.4)
Total current liabilities	(40.8)	(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	(40.8)	(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	(1.1)	(1.0)
Accumulated other comprehensive income (loss) (AOCI)	0.0	0.0
Total stockholders' equity	51.0	51.1
Total liabilities and stockholders' equity	10.2	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