Guarantee of Securities of Subsidiaries Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Revenues:
Transportation									$ 1,180.7	$ 1,142.4	$ 1,091.1
Parking and lending									20.2	18.2	11.4
Storage									81.5	91.4	81.3
Other									40.2	55.2	65.4
Total operating revenues	$ 337.5	$ 300.5	$ 317.6	$ 367.0	$ 352.6	$ 303.3	$ 306.3	$ 345.0	1,322.6	1,307.2	1,249.2
Operating Costs and Expenses:
Fuel and transportation									54.8	70.8	99.3
Operation and maintenance									204.2	199.9	209.5
Administrative and general									126.5	142.2	130.4
Other operating costs and expenses									470.6	416.8	414.2
Total operating costs and expenses	213.8	189.7	250.4	202.2	219.7	209.6	197.0	203.4	856.1	829.7	853.4
Operating (loss) income	123.7	110.8	67.2	164.8	132.9	93.7	109.3	141.6	466.5	477.5	395.8
Other Deductions (Income):
Interest expense									171.0	182.8	176.4
Interest expense - affiliates									0.0	0.0	0.0
Interest income									(0.4)	(0.4)	(0.4)
Equity in earnings of subsidiaries									0.0	0.0	0.0
Miscellaneous other income, net									(2.1)	(7.7)	(2.7)
Total other (income) deductions									168.5	174.7	173.3
Income (loss) before income taxes	84.3	70.1	24.1	119.5	88.4	47.3	65.9	101.2	298.0	302.8	222.5
Income taxes	0.1	0.3	0.4	0.2	0.2	0.0	0.2	0.2	1.0	0.6	0.5
Net income (loss)	$ 84.2	$ 69.8	$ 23.7	$ 119.3	$ 88.2	$ 47.3	$ 65.7	$ 101.0	297.0	302.2	222.0
Reportable Legal Entities | Parent Guarantor
Operating Revenues:
Transportation									0.0	0.0	0.0
Parking and lending									0.0	0.0	0.0
Storage									0.0	0.0	0.0
Other									0.0	0.0	0.0
Total operating revenues									0.0	0.0	0.0
Operating Costs and Expenses:
Fuel and transportation									0.0	0.0	0.0
Operation and maintenance									0.0	0.0	0.0
Administrative and general									(0.3)	0.5	0.0
Other operating costs and expenses									0.6	0.4	0.3
Total operating costs and expenses									0.3	0.9	0.3
Operating (loss) income									(0.3)	(0.9)	(0.3)
Other Deductions (Income):
Interest expense									0.0	0.0	0.0
Interest Income - affiliates									(47.3)	(37.8)	(28.8)
Interest income									0.0	0.0	0.0
Equity in earnings of subsidiaries									(250.0)	(265.5)	(193.5)
Miscellaneous other income, net									0.0	0.2	0.0
Total other (income) deductions									(297.3)	(303.1)	(222.3)
Income (loss) before income taxes									297.0	302.2	222.0
Income taxes									0.0	0.0	0.0
Net income (loss)									297.0	302.2	222.0
Reportable Legal Entities | Subsidiary Issuer
Operating Revenues:
Transportation									0.0	0.0	0.0
Parking and lending									0.0	0.0	0.0
Storage									0.0	0.0	0.0
Other									0.0	0.0	0.0
Total operating revenues									0.0	0.0	0.0
Operating Costs and Expenses:
Fuel and transportation									0.0	0.0	0.0
Operation and maintenance									0.0	0.0	0.0
Administrative and general									0.0	0.0	0.0
Other operating costs and expenses									0.0	0.0	0.0
Total operating costs and expenses									0.0	0.0	0.0
Operating (loss) income									0.0	0.0	0.0
Other Deductions (Income):
Interest expense									129.6	123.8	104.0
Interest expense - affiliates									39.9	44.4	38.2
Interest income									(0.2)	(0.1)	0.0
Equity in earnings of subsidiaries									(419.3)	(433.6)	(335.7)
Miscellaneous other income, net									0.0	0.0	0.0
Total other (income) deductions									(250.0)	(265.5)	(193.5)
Income (loss) before income taxes									250.0	265.5	193.5
Income taxes									0.0	0.0	0.0
Net income (loss)									250.0	265.5	193.5
Reportable Legal Entities | Non-Guarantor Subsidiaries
Operating Revenues:
Transportation									1,269.0	1,230.2	1,178.5
Parking and lending									20.5	20.1	11.6
Storage									81.5	91.4	81.3
Other									40.2	55.2	65.4
Total operating revenues									1,411.2	1,396.9	1,336.8
Operating Costs and Expenses:
Fuel and transportation									143.4	160.5	186.9
Operation and maintenance									204.2	199.9	209.5
Administrative and general									126.8	141.7	130.4
Other operating costs and expenses									470.0	416.4	413.9
Total operating costs and expenses									944.4	918.5	940.7
Operating (loss) income									466.8	478.4	396.1
Other Deductions (Income):
Interest expense									41.4	59.0	72.4
Interest Income - affiliates										(6.6)	(9.4)
Interest expense - affiliates									7.4
Interest income									(0.2)	(0.3)	(0.4)
Equity in earnings of subsidiaries									0.0	0.0	0.0
Miscellaneous other income, net									(2.1)	(7.9)	(2.7)
Total other (income) deductions									46.5	44.2	59.9
Income (loss) before income taxes									420.3	434.2	336.2
Income taxes									1.0	0.6	0.5
Net income (loss)									419.3	433.6	335.7
Eliminations
Operating Revenues:
Transportation									(88.3)	(87.8)	(87.4)
Parking and lending									(0.3)	(1.9)	(0.2)
Storage									0.0	0.0	0.0
Other									0.0	0.0	0.0
Total operating revenues									(88.6)	(89.7)	(87.6)
Operating Costs and Expenses:
Fuel and transportation									(88.6)	(89.7)	(87.6)
Operation and maintenance									0.0	0.0	0.0
Administrative and general									0.0	0.0	0.0
Other operating costs and expenses									0.0	0.0	0.0
Total operating costs and expenses									(88.6)	(89.7)	(87.6)
Operating (loss) income									0.0	0.0	0.0
Other Deductions (Income):
Interest expense									0.0	0.0	0.0
Interest expense - affiliates									0.0	0.0	0.0
Interest income									0.0	0.0	0.0
Equity in earnings of subsidiaries									669.3	699.1	529.2
Miscellaneous other income, net									0.0	0.0	0.0
Total other (income) deductions									669.3	699.1	529.2
Income (loss) before income taxes									(669.3)	(699.1)	(529.2)
Income taxes									0.0	0.0	0.0
Net income (loss)									$ (669.3)	$ (699.1)	$ (529.2)