Guarantee of Securities of Subsidiaries Guarantee of Securities of Subsidiaries Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Operating Revenues:
Transportation	$ 259.6	$ 255.6	$ 835.3	$ 802.1
Parking and lending	4.9	2.3	13.4	7.9
Storage	23.4	20.4	68.0	60.6
Other	15.4	15.8	37.9	51.8
Total operating revenues	303.3	294.1	954.6	922.4
Operating Costs and Expenses:
Fuel and transportation	19.1	25.1	51.0	78.0
Operation and maintenance	52.1	54.0	143.8	148.7
Administrative and general	34.4	31.7	104.6	94.1
Other operating costs and expenses	104.0	102.6	310.6	312.2
Total operating costs and expenses	209.6	213.4	610.0	633.0
Operating (loss) income	93.7	80.7	344.6	289.4
Other Deductions (Income):
Interest expense	48.4	43.1	136.4	134.2
Interest expense - affiliates, net	0.0	0.0	0.0	0.0
Interest income	(0.1)	(0.1)	(0.3)	(0.3)
Equity in earnings of subsidiaries	0.0	0.0	0.0	0.0
Miscellaneous other income, net	(1.9)	(0.7)	(5.9)	(1.3)
Total other (income) deductions	46.4	42.3	130.2	132.6
Income (loss) before income taxes	47.3	38.4	214.4	156.8
Income taxes	0.0	0.1	0.4	0.4
Net income (loss)	47.3	38.3	214.0	156.4
Reportable Legal Entities | Parent Guarantor
Operating Revenues:
Transportation	0.0	0.0	0.0	0.0
Parking and lending	0.0	0.0	0.0	0.0
Storage	0.0	0.0	0.0	0.0
Other	0.0	0.0	0.0	0.0
Total operating revenues	0.0	0.0	0.0	0.0
Operating Costs and Expenses:
Fuel and transportation	0.0	0.0	0.0	0.0
Operation and maintenance	0.0	0.0	0.0	0.0
Administrative and general	0.0	0.2	0.1	0.0
Other operating costs and expenses	0.1	0.0	0.3	0.2
Total operating costs and expenses	0.1	0.2	0.4	0.2
Operating (loss) income	(0.1)	(0.2)	(0.4)	(0.2)
Other Deductions (Income):
Interest expense	0.0	0.0	0.0	0.0
Interest income - affiliates, net	(9.7)	(7.3)	(27.6)	(21.2)
Interest income	0.0	0.0	0.0	0.0
Equity in earnings of subsidiaries	(37.7)	(31.2)	(187.0)	(135.4)
Miscellaneous other income, net	0.0	0.0	0.2	0.0
Total other (income) deductions	(47.4)	(38.5)	(214.4)	(156.6)
Income (loss) before income taxes	47.3	38.3	214.0	156.4
Income taxes	0.0	0.0	0.0	0.0
Net income (loss)	47.3	38.3	214.0	156.4
Reportable Legal Entities | Subsidiary Issuer
Operating Revenues:
Transportation	0.0	0.0	0.0	0.0
Parking and lending	0.0	0.0	0.0	0.0
Storage	0.0	0.0	0.0	0.0
Other	0.0	0.0	0.0	0.0
Total operating revenues	0.0	0.0	0.0	0.0
Operating Costs and Expenses:
Fuel and transportation	0.0	0.0	0.0	0.0
Operation and maintenance	0.0	0.0	0.0	0.0
Administrative and general	0.0	0.0	0.0	0.0
Other operating costs and expenses	0.0	0.0	0.0	0.0
Total operating costs and expenses	0.0	0.0	0.0	0.0
Operating (loss) income	0.0	0.0	0.0	0.0
Other Deductions (Income):
Interest expense	34.6	26.1	91.1	77.9
Interest expense - affiliates, net	11.7	9.7	35.2	28.3
Interest income	0.0	0.0	0.0	0.0
Equity in earnings of subsidiaries	(84.0)	(67.0)	(313.3)	(241.6)
Miscellaneous other income, net	0.0	0.0	0.0	0.0
Total other (income) deductions	(37.7)	(31.2)	(187.0)	(135.4)
Income (loss) before income taxes	37.7	31.2	187.0	135.4
Income taxes	0.0	0.0	0.0	0.0
Net income (loss)	37.7	31.2	187.0	135.4
Reportable Legal Entities | Non-Guarantor Subsidiaries
Operating Revenues:
Transportation	281.9	277.1	901.0	867.3
Parking and lending	5.1	2.3	14.6	8.1
Storage	23.4	20.4	68.0	60.6
Other	15.4	15.8	37.9	51.8
Total operating revenues	325.8	315.6	1,021.5	987.8
Operating Costs and Expenses:
Fuel and transportation	41.6	46.6	117.9	143.4
Operation and maintenance	52.1	54.0	143.8	148.7
Administrative and general	34.4	31.5	104.5	94.1
Other operating costs and expenses	103.9	102.6	310.3	312.0
Total operating costs and expenses	232.0	234.7	676.5	698.2
Operating (loss) income	93.8	80.9	345.0	289.6
Other Deductions (Income):
Interest expense	13.8	17.0	45.3	56.3
Interest income - affiliates, net	(2.0)	(2.4)	(7.6)	(7.1)
Interest income	(0.1)	(0.1)	(0.3)	(0.3)
Equity in earnings of subsidiaries	0.0	0.0	0.0	0.0
Miscellaneous other income, net	(1.9)	(0.7)	(6.1)	(1.3)
Total other (income) deductions	9.8	13.8	31.3	47.6
Income (loss) before income taxes	84.0	67.1	313.7	242.0
Income taxes	0.0	0.1	0.4	0.4
Net income (loss)	84.0	67.0	313.3	241.6
Eliminations
Operating Revenues:
Transportation	(22.3)	(21.5)	(65.7)	(65.2)
Parking and lending	(0.2)	0.0	(1.2)	(0.2)
Storage	0.0	0.0	0.0	0.0
Other	0.0	0.0	0.0	0.0
Total operating revenues	(22.5)	(21.5)	(66.9)	(65.4)
Operating Costs and Expenses:
Fuel and transportation	(22.5)	(21.5)	(66.9)	(65.4)
Operation and maintenance	0.0	0.0	0.0	0.0
Administrative and general	0.0	0.0	0.0	0.0
Other operating costs and expenses	0.0	0.0	0.0	0.0
Total operating costs and expenses	(22.5)	(21.5)	(66.9)	(65.4)
Operating (loss) income	0.0	0.0	0.0	0.0
Other Deductions (Income):
Interest expense	0.0	0.0	0.0	0.0
Interest expense - affiliates, net	0.0	0.0	0.0	0.0
Interest income	0.0	0.0	0.0	0.0
Equity in earnings of subsidiaries	121.7	98.2	500.3	377.0
Miscellaneous other income, net	0.0	0.0	0.0	0.0
Total other (income) deductions	121.7	98.2	500.3	377.0
Income (loss) before income taxes	(121.7)	(98.2)	(500.3)	(377.0)
Income taxes	0.0	0.0	0.0	0.0
Net income (loss)	$ (121.7)	$ (98.2)	$ (500.3)	$ (377.0)