Guarantor Subsidiaries (Consolidating Income Statement and Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Operating Revenues
Total operating revenues	$ 3,184.8	$ 3,116.4
Operating Expenses
Depreciation and amortization	39.4	39.7
Selling, general and administrative	39.2	34.6
Accretion of asset retirement obligations	0.6	0.5	$ 2.1
Total operating expenses	3,054.1	3,097.8
Net settlement proceeds	0.0	0.1
Gain (loss) on sale of assets	0.1	(0.1)
Income (loss) from operations	130.8	18.6
Other income (expense)
Interest income	0.8	0.7
Interest expense	(13.3)	(13.6)
Other nonoperating income (expense)	(1.0)	0.2
Total other income (expense)	(13.5)	(12.7)
Income (loss) before income taxes	117.3	5.9
Income tax expense (benefit)	28.0	0.6
Income (loss)	89.3	5.3
Equity earnings in affiliates, net of tax	0.0	0.0
Net Income (Loss) Attributable to Parent, Total	89.3	5.3
Other comprehensive income (loss)	(2.7)	0.0
Comprehensive income (loss)	86.6	5.3
Eliminations
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Depreciation and amortization	0.0	0.0
Selling, general and administrative	0.0	0.0
Accretion of asset retirement obligations	0.0	0.0
Total operating expenses	0.0	0.0
Net settlement proceeds		0.0
Gain (loss) on sale of assets	0.0	0.0
Income (loss) from operations	0.0	0.0
Other income (expense)
Interest income	0.0	0.0
Interest expense	0.0	0.0
Other nonoperating income (expense)	0.0	0.0
Total other income (expense)	0.0	0.0
Income (loss) before income taxes	0.0	0.0
Income tax expense (benefit)	0.0	0.0
Income (loss)	0.0	0.0
Equity earnings in affiliates, net of tax	(89.2)	(5.1)
Net Income (Loss) Attributable to Parent, Total	(89.2)	(5.1)
Other comprehensive income (loss)	0.0	0.0
Comprehensive income (loss)	(89.2)	(5.1)
Parent Company | Reportable Legal Entities
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Depreciation and amortization	0.0	0.0
Selling, general and administrative	0.0	0.0
Accretion of asset retirement obligations	0.0	0.0
Total operating expenses	0.0	0.0
Net settlement proceeds		0.0
Gain (loss) on sale of assets	0.0	0.0
Income (loss) from operations	0.0	0.0
Other income (expense)
Interest income	0.0	0.0
Interest expense	0.0	0.0
Other nonoperating income (expense)	0.0	0.0
Total other income (expense)	0.0	0.0
Income (loss) before income taxes	0.0	0.0
Income tax expense (benefit)	0.0	0.0
Income (loss)	0.0	0.0
Equity earnings in affiliates, net of tax	89.3	5.3
Net Income (Loss) Attributable to Parent, Total	89.3	5.3
Other comprehensive income (loss)	0.0	0.0
Comprehensive income (loss)	89.3	5.3
Issuer | Reportable Legal Entities
Operating Revenues
Total operating revenues	3,184.8	3,116.4
Operating Expenses
Depreciation and amortization	39.4	39.7
Selling, general and administrative	39.2	34.6
Accretion of asset retirement obligations	0.6	0.5
Total operating expenses	3,054.1	3,097.8
Net settlement proceeds		0.1
Gain (loss) on sale of assets	0.1	(0.1)
Income (loss) from operations	130.8	18.6
Other income (expense)
Interest income	0.8	0.7
Interest expense	(13.3)	(13.6)
Other nonoperating income (expense)	(0.9)	0.4
Total other income (expense)	(13.4)	(12.5)
Income (loss) before income taxes	117.4	6.1
Income tax expense (benefit)	28.0	0.6
Income (loss)	89.4	5.5
Equity earnings in affiliates, net of tax	(0.1)	(0.2)
Net Income (Loss) Attributable to Parent, Total	89.3	5.3
Other comprehensive income (loss)	(2.7)	0.0
Comprehensive income (loss)	86.6	5.3
Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Depreciation and amortization	0.0	0.0
Selling, general and administrative	0.0	0.0
Accretion of asset retirement obligations	0.0	0.0
Total operating expenses	0.0	0.0
Net settlement proceeds		0.0
Gain (loss) on sale of assets	0.0	0.0
Income (loss) from operations	0.0	0.0
Other income (expense)
Interest income	0.0	0.0
Interest expense	0.0	0.0
Other nonoperating income (expense)	(0.1)	(0.2)
Total other income (expense)	(0.1)	(0.2)
Income (loss) before income taxes	(0.1)	(0.2)
Income tax expense (benefit)	0.0	0.0
Income (loss)	(0.1)	(0.2)
Equity earnings in affiliates, net of tax	0.0	0.0
Net Income (Loss) Attributable to Parent, Total	(0.1)	(0.2)
Other comprehensive income (loss)	0.0	0.0
Comprehensive income (loss)	(0.1)	(0.2)
Non-Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Depreciation and amortization	0.0	0.0
Selling, general and administrative	0.0	0.0
Accretion of asset retirement obligations	0.0	0.0
Total operating expenses	0.0	0.0
Net settlement proceeds		0.0
Gain (loss) on sale of assets	0.0	0.0
Income (loss) from operations	0.0	0.0
Other income (expense)
Interest income	0.0	0.0
Interest expense	0.0	0.0
Other nonoperating income (expense)	0.0	0.0
Total other income (expense)	0.0	0.0
Income (loss) before income taxes	0.0	0.0
Income tax expense (benefit)	0.0	0.0
Income (loss)	0.0	0.0
Equity earnings in affiliates, net of tax	0.0	0.0
Net Income (Loss) Attributable to Parent, Total	0.0	0.0
Other comprehensive income (loss)	0.0	0.0
Comprehensive income (loss)	0.0	0.0
Product
Operating Revenues
Total operating revenues	2,480.2	2,499.8
Operating Expenses
Operating expenses	2,259.8	2,381.5
Product | Eliminations
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Operating expenses	0.0	0.0
Product | Parent Company | Reportable Legal Entities
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Operating expenses	0.0	0.0
Product | Issuer | Reportable Legal Entities
Operating Revenues
Total operating revenues	2,480.2	2,499.8
Operating Expenses
Operating expenses	2,259.8	2,381.5
Product | Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Operating expenses	0.0	0.0
Product | Non-Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Operating expenses	0.0	0.0
Merchandise sales
Operating Revenues
Total operating revenues	687.5	606.2
Operating Expenses
Operating expenses	580.0	508.7
Merchandise sales | Eliminations
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Operating expenses	0.0	0.0
Merchandise sales | Parent Company | Reportable Legal Entities
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Operating expenses	0.0	0.0
Merchandise sales | Issuer | Reportable Legal Entities
Operating Revenues
Total operating revenues	687.5	606.2
Operating Expenses
Operating expenses	580.0	508.7
Merchandise sales | Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Operating expenses	0.0	0.0
Merchandise sales | Non-Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Operating expenses	0.0	0.0
Other operating revenues
Operating Revenues
Total operating revenues	17.1	10.4
Operating Expenses
Operating expenses	135.1	132.8
Other operating revenues | Eliminations
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Operating expenses	0.0	0.0
Other operating revenues | Parent Company | Reportable Legal Entities
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Operating expenses	0.0	0.0
Other operating revenues | Issuer | Reportable Legal Entities
Operating Revenues
Total operating revenues	17.1	10.4
Operating Expenses
Operating expenses	135.1	132.8
Other operating revenues | Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Operating expenses	0.0	0.0
Other operating revenues | Non-Guarantor Subsidiaries | Reportable Legal Entities
Operating Revenues
Total operating revenues	0.0	0.0
Operating Expenses
Operating expenses	$ 0.0	$ 0.0