Guarantor Subsidiaries (Consolidating Balance Sheet) (Details) - USD ($) $ / shares in Units, $ 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, at cost less accumulated depreciation and amortization	1,834.3		1,807.3
Investments in subsidiaries	0.0		0.0
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
Inter-company accounts payable	0.0		0.0
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	0.0		0.0
Common Stock	0.5		0.5
Treasury Stock	(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
Allowance for doubtful accounts	0.1		1.2
Property, plant and equipment, accumulated depreciation and amortization	$ 1,143.6		$ 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)	17,589,533		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	(2,993.6)		(2,735.7)
Other assets	0.0		0.0
Total assets	(2,993.6)		(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	(1,625.1)		(1,367.2)
Accumulated other comprehensive income (loss) (AOCI)	0.0		0.0
Total stockholders' equity	(2,993.6)		(2,735.7)
Total liabilities and stockholders' equity	(2,993.6)		(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	2,850.0		2,591.8
Other assets	0.0		0.0
Total assets	2,850.0		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	2,760.6		2,502.4
Accumulated other comprehensive income (loss) (AOCI)	0.0		0.0
Total stockholders' equity	2,709.0		2,591.9
Total liabilities and stockholders' equity	2,850.0		2,591.8
Issuer | Reportable Legal Entities
Current assets
Cash and cash equivalents	403.0		279.4
Accounts receivable—trade, less allowance for doubtful accounts	161.1		173.0
Inventories, at lower of cost or market	257.7		227.6
Prepaid expenses and other current assets	17.7		29.6
Total current assets	839.5		709.6
Property, plant and equipment, at cost less accumulated depreciation and amortization	1,823.3		1,799.1
Investments in subsidiaries	143.6		143.9
Other assets	186.1		169.1
Total assets	2,992.5		2,821.7
Current liabilities
Current maturities of long-term debt	51.2		38.8
Inter-company accounts payable	52.4		196.1
Trade accounts payable and accrued liabilities	461.2		466.2
Income taxes payable	41.2
Total current liabilities	606.0		701.1
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	1,995.0		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)	578.8		578.8
Retained earnings	421.2		161.9
Accumulated other comprehensive income (loss) (AOCI)	(2.5)		0.7
Total stockholders' equity	997.5		741.4
Total liabilities and stockholders' equity	2,992.5		2,821.7
Guarantor Subsidiaries | Reportable Legal Entities
Current assets
Cash and cash equivalents	0.6		0.9
Accounts receivable—trade, less allowance for doubtful accounts	0.2		(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	0.8		1.2
Property, plant and equipment, at cost less accumulated depreciation and amortization	11.0		8.2
Investments in subsidiaries	0.0		0.0
Other assets	0.0		0.0
Total assets	11.8		9.4
Current liabilities
Current maturities of long-term debt	0.0		0.0
Inter-company accounts payable	(39.1)		(41.7)
Trade accounts payable and accrued liabilities	0.5		0.0
Income taxes payable	(0.4)
Total current liabilities	(39.0)		(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	(39.0)		(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	(1.3)		(1.0)
Accumulated other comprehensive income (loss) (AOCI)	0.0		0.0
Total stockholders' equity	50.8		51.1
Total liabilities and stockholders' equity	11.8		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