Guarantee of Securities of Subsidiaries Guarantee of Securities of Subsidiaries Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating Revenues:
Transportation	$ 306.4	$ 298.5
Storage, parking and lending	23.4	25.1
Other	16.1	11.8
Total operating revenues	345.9	335.4
Operating Costs and Expenses:
Fuel and transportation	2.6	4.4
Operation and maintenance	43.0	46.4
Administrative and general	33.2	32.1
Other operating costs and expenses	114.0	111.8
Total operating costs and expenses	192.8	194.7
Operating (loss) income	153.1	140.7
Other Deductions (Income):
Interest expense	45.3	44.1
Interest expense - affiliates, net	0.0	0.0
Interest income	(0.3)	0.0
Equity in earnings of subsidiaries	0.0	0.0
Miscellaneous other income, net	(0.2)	(0.8)
Total other (income) deductions	44.8	43.3
Income (loss) before income taxes	108.3	97.4
Income taxes	0.2	0.2
Net income (loss)	108.1	97.2
Reportable Legal Entities | Parent Guarantor
Operating Revenues:
Transportation	0.0	0.0
Storage, parking and lending	0.0	0.0
Other	0.0	0.0
Total operating revenues	0.0	0.0
Operating Costs and Expenses:
Fuel and transportation	0.0	0.0
Operation and maintenance	0.0	0.0
Administrative and general	0.0	0.0
Other operating costs and expenses	0.1	0.0
Total operating costs and expenses	0.1	0.0
Operating (loss) income	(0.1)	0.0
Other Deductions (Income):
Interest expense	0.0	0.0
Interest income - affiliates, net	(19.0)	(14.3)
Interest income	0.0
Equity in earnings of subsidiaries	(89.2)	(82.9)
Miscellaneous other income, net	0.0	0.0
Total other (income) deductions	(108.2)	(97.2)
Income (loss) before income taxes	108.1	97.2
Income taxes	0.0	0.0
Net income (loss)	108.1	97.2
Reportable Legal Entities | Subsidiary Issuer
Operating Revenues:
Transportation	0.0	0.0
Storage, parking and lending	0.0	0.0
Other	0.0	0.0
Total operating revenues	0.0	0.0
Operating Costs and Expenses:
Fuel and transportation	0.0	0.0
Operation and maintenance	0.0	0.0
Administrative and general	0.0	0.0
Other operating costs and expenses	0.0	0.0
Total operating costs and expenses	0.0	0.0
Operating (loss) income	0.0	0.0
Other Deductions (Income):
Interest expense	29.5	31.9
Interest expense - affiliates, net	19.1	10.8
Interest income	0.0
Equity in earnings of subsidiaries	(137.8)	(125.6)
Miscellaneous other income, net	0.0	0.0
Total other (income) deductions	(89.2)	(82.9)
Income (loss) before income taxes	89.2	82.9
Income taxes	0.0	0.0
Net income (loss)	89.2	82.9
Reportable Legal Entities | Non-Guarantor Subsidiaries
Operating Revenues:
Transportation	326.6	319.3
Storage, parking and lending	23.9	25.4
Other	16.1	11.8
Total operating revenues	366.6	356.5
Operating Costs and Expenses:
Fuel and transportation	23.3	25.5
Operation and maintenance	43.0	46.4
Administrative and general	33.2	32.1
Other operating costs and expenses	113.9	111.8
Total operating costs and expenses	213.4	215.8
Operating (loss) income	153.2	140.7
Other Deductions (Income):
Interest expense	15.8	12.2
Interest income - affiliates, net	(0.1)
Interest expense - affiliates, net		3.5
Interest income	(0.3)
Equity in earnings of subsidiaries	0.0	0.0
Miscellaneous other income, net	(0.2)	(0.8)
Total other (income) deductions	15.2	14.9
Income (loss) before income taxes	138.0	125.8
Income taxes	0.2	0.2
Net income (loss)	137.8	125.6
Eliminations
Operating Revenues:
Transportation	(20.2)	(20.8)
Storage, parking and lending	(0.5)	(0.3)
Other	0.0	0.0
Total operating revenues	(20.7)	(21.1)
Operating Costs and Expenses:
Fuel and transportation	(20.7)	(21.1)
Operation and maintenance	0.0	0.0
Administrative and general	0.0	0.0
Other operating costs and expenses	0.0	0.0
Total operating costs and expenses	(20.7)	(21.1)
Operating (loss) income	0.0	0.0
Other Deductions (Income):
Interest expense	0.0	0.0
Interest expense - affiliates, net	0.0	0.0
Interest income	0.0
Equity in earnings of subsidiaries	227.0	208.5
Miscellaneous other income, net	0.0	0.0
Total other (income) deductions	227.0	208.5
Income (loss) before income taxes	(227.0)	(208.5)
Income taxes	0.0	0.0
Net income (loss)	$ (227.0)	$ (208.5)