Guarantor Subsidiaries (Consolidating Income Statement and Comprehensive Income) (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended		12 Months Ended
		Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Operating Revenues
Total operating revenues		$ 2,379.6		$ 3,800.4		$ 5,564.4	$ 6,916.8
Operating Expenses
Depreciation and amortization		39.5		36.5		78.9	76.2
Selling, general and administrative		37.1		35.1		76.3	69.7
Accretion of asset retirement obligations		0.5		0.5		1.1	1.0	$ 2.1
Total operating expenses		2,145.4		3,744.7		5,199.5	6,842.5
Net settlement proceeds		0.0		0.0		0.0	0.1
Gain (loss) on sale of assets		1.3		0.0		1.4	(0.1)
Income (loss) from operations		235.5		55.7		366.3	74.3
Other income (expense)
Interest income		0.2		0.9		1.0	1.6
Interest expense		(13.0)		(13.2)		(26.3)	(26.8)
Other nonoperating income (expense)		0.3		(0.1)		(0.7)	0.1
Total other income (expense)		(12.5)		(12.4)		(26.0)	(25.1)
Income (loss) before income taxes		223.0		43.3		340.3	49.2
Income tax expense (benefit)		54.1		10.6		82.1	11.2
Income (loss)		168.9		32.7		258.2	38.0
Equity earnings in affiliates, net of tax		0.0		0.0		0.0	0.0
Net Income		168.9	$ 89.3	32.7	$ 5.3	258.2	38.0
Other comprehensive income (loss)		(0.5)		0.0		(3.2)	0.0
Comprehensive income		168.4		32.7		255.0	38.0
Eliminations
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Depreciation and amortization		0.0		0.0		0.0	0.0
Selling, general and administrative		0.0		0.0		0.0	0.0
Accretion of asset retirement obligations		0.0		0.0		0.0	0.0
Total operating expenses		0.0		0.0		0.0	0.0
Net settlement proceeds		0.0		0.0			0.0
Gain (loss) on sale of assets		0.0		0.0		0.0	0.0
Income (loss) from operations		0.0		0.0		0.0	0.0
Other income (expense)
Interest income		0.0		0.0		0.0	0.0
Interest expense		0.0		0.0		0.0	0.0
Other nonoperating income (expense)		0.0		0.0		0.0	0.0
Total other income (expense)		0.0		0.0		0.0	0.0
Income (loss) before income taxes		0.0		0.0		0.0	0.0
Income tax expense (benefit)		0.0		0.0		0.0	0.0
Income (loss)		0.0		0.0		0.0	0.0
Equity earnings in affiliates, net of tax		(168.7)		(32.6)		(257.9)	(37.7)
Net Income		(168.7)		(32.6)		(257.9)	(37.7)
Other comprehensive income (loss)		0.0		0.0		0.0	0.0
Comprehensive income		(168.7)		(32.6)		(257.9)	(37.7)
Parent Company | Reportable Legal Entities
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Depreciation and amortization		0.0		0.0		0.0	0.0
Selling, general and administrative		0.0		0.0		0.0	0.0
Accretion of asset retirement obligations		0.0		0.0		0.0	0.0
Total operating expenses		0.0		0.0		0.0	0.0
Net settlement proceeds		0.0		0.0			0.0
Gain (loss) on sale of assets		0.0		0.0		0.0	0.0
Income (loss) from operations		0.0		0.0		0.0	0.0
Other income (expense)
Interest income		0.0		0.0		0.0	0.0
Interest expense		0.0		0.0		0.0	0.0
Other nonoperating income (expense)		0.0		0.0		0.0	0.0
Total other income (expense)		0.0		0.0		0.0	0.0
Income (loss) before income taxes		0.0		0.0		0.0	0.0
Income tax expense (benefit)		0.0		0.0		0.0	0.0
Income (loss)		0.0		0.0		0.0	0.0
Equity earnings in affiliates, net of tax		168.9		32.7		258.2	38.0
Net Income		168.9		32.7		258.2	38.0
Other comprehensive income (loss)		0.0		0.0		0.0	0.0
Comprehensive income		168.9		32.7		258.2	38.0
Issuer | Reportable Legal Entities
Operating Revenues
Total operating revenues		2,379.6		3,800.4		5,564.4	6,916.8
Operating Expenses
Depreciation and amortization		39.4		36.5		78.8	76.2
Selling, general and administrative		37.1		35.1		76.3	69.7
Accretion of asset retirement obligations		0.5		0.5		1.1	1.0
Total operating expenses		2,145.3		3,744.7		5,199.4	6,842.5
Net settlement proceeds		0.0		0.0			0.1
Gain (loss) on sale of assets		1.3		0.0		1.4	(0.1)
Income (loss) from operations		235.6		55.7		366.4	74.3
Other income (expense)
Interest income		0.2		0.9		1.0	1.6
Interest expense		(13.0)		(13.2)		(26.3)	(26.8)
Other nonoperating income (expense)		0.5		0.1		(0.4)	0.5
Total other income (expense)		(12.3)		(12.2)		(25.7)	(24.7)
Income (loss) before income taxes		223.3		43.5		340.7	49.6
Income tax expense (benefit)		54.2		10.7		82.2	11.3
Income (loss)		169.1		32.8		258.5	38.3
Equity earnings in affiliates, net of tax		(0.2)		(0.1)		(0.3)	(0.3)
Net Income		168.9		32.7		258.2	38.0
Other comprehensive income (loss)		(0.5)		0.0		(3.2)	0.0
Comprehensive income		168.4		32.7		255.0	38.0
Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Depreciation and amortization		0.1		0.0		0.1	0.0
Selling, general and administrative		0.0		0.0		0.0	0.0
Accretion of asset retirement obligations		0.0		0.0		0.0	0.0
Total operating expenses		0.1		0.0		0.1	0.0
Net settlement proceeds		0.0		0.0			0.0
Gain (loss) on sale of assets		0.0		0.0		0.0	0.0
Income (loss) from operations		(0.1)		0.0		(0.1)	0.0
Other income (expense)
Interest income		0.0		0.0		0.0	0.0
Interest expense		0.0		0.0		0.0	0.0
Other nonoperating income (expense)		(0.2)		(0.2)		(0.3)	(0.4)
Total other income (expense)		(0.2)		(0.2)		(0.3)	(0.4)
Income (loss) before income taxes		(0.3)		(0.2)		(0.4)	(0.4)
Income tax expense (benefit)		(0.1)		(0.1)		(0.1)	(0.1)
Income (loss)		(0.2)		(0.1)		(0.3)	(0.3)
Equity earnings in affiliates, net of tax		0.0		0.0		0.0	0.0
Net Income		(0.2)		(0.1)		(0.3)	(0.3)
Other comprehensive income (loss)		0.0		0.0		0.0	0.0
Comprehensive income		(0.2)		(0.1)		(0.3)	(0.3)
Non-Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Depreciation and amortization		0.0		0.0		0.0	0.0
Selling, general and administrative		0.0		0.0		0.0	0.0
Accretion of asset retirement obligations		0.0		0.0		0.0	0.0
Total operating expenses		0.0		0.0		0.0	0.0
Net settlement proceeds		0.0		0.0			0.0
Gain (loss) on sale of assets		0.0		0.0		0.0	0.0
Income (loss) from operations		0.0		0.0		0.0	0.0
Other income (expense)
Interest income		0.0		0.0		0.0	0.0
Interest expense		0.0		0.0		0.0	0.0
Other nonoperating income (expense)		0.0		0.0		0.0	0.0
Total other income (expense)		0.0		0.0		0.0	0.0
Income (loss) before income taxes		0.0		0.0		0.0	0.0
Income tax expense (benefit)		0.0		0.0		0.0	0.0
Income (loss)		0.0		0.0		0.0	0.0
Equity earnings in affiliates, net of tax		0.0		0.0		0.0	0.0
Net Income		0.0		0.0		0.0	0.0
Other comprehensive income (loss)		0.0		0.0		0.0	0.0
Comprehensive income		0.0		0.0		0.0	0.0
Product
Operating Revenues
Total operating revenues	[1]	1,588.9		3,129.7		4,069.1	5,629.5
Operating Expenses
Operating expenses	[1]	1,287.8		2,973.7		3,547.6	5,355.2
Product | Eliminations
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Operating expenses		0.0		0.0		0.0	0.0
Product | Parent Company | Reportable Legal Entities
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Operating expenses		0.0		0.0		0.0	0.0
Product | Issuer | Reportable Legal Entities
Operating Revenues
Total operating revenues		1,588.9		3,129.7		4,069.1	5,629.5
Operating Expenses
Operating expenses		1,287.8		2,973.7		3,547.6	5,355.2
Product | Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Operating expenses		0.0		0.0		0.0	0.0
Product | Non-Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Operating expenses		0.0		0.0		0.0	0.0
Merchandise sales
Operating Revenues
Total operating revenues		767.1		658.8		1,454.6	1,265.0
Operating Expenses
Operating expenses		648.7		553.3		1,228.7	1,062.0
Merchandise sales | Eliminations
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Operating expenses		0.0		0.0		0.0	0.0
Merchandise sales | Parent Company | Reportable Legal Entities
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Operating expenses		0.0		0.0		0.0	0.0
Merchandise sales | Issuer | Reportable Legal Entities
Operating Revenues
Total operating revenues		767.1		658.8		1,454.6	1,265.0
Operating Expenses
Operating expenses		648.7		553.3		1,228.7	1,062.0
Merchandise sales | Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Operating expenses		0.0		0.0		0.0	0.0
Merchandise sales | Non-Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Operating expenses		0.0		0.0		0.0	0.0
Other operating revenues
Operating Revenues
Total operating revenues		23.6		11.9		40.7	22.3
Operating Expenses
Operating expenses		131.8		145.6		266.9	278.4
Other operating revenues | Eliminations
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Operating expenses		0.0		0.0		0.0	0.0
Other operating revenues | Parent Company | Reportable Legal Entities
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Operating expenses		0.0		0.0		0.0	0.0
Other operating revenues | Issuer | Reportable Legal Entities
Operating Revenues
Total operating revenues		23.6		11.9		40.7	22.3
Operating Expenses
Operating expenses		131.8		145.6		266.9	278.4
Other operating revenues | Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Operating expenses		0.0		0.0		0.0	0.0
Other operating revenues | Non-Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues		0.0		0.0		0.0	0.0
Operating Expenses
Operating expenses		$ 0.0		$ 0.0		$ 0.0	$ 0.0

[1] Includes excise taxes of $853.7 million for the three months ended June 30, 2020, $498.3 million for the three months ended June 30, 2019, $473.5 million for the six months ended June 30, 2020 and $953.6 million the six months ended June 30, 2019.