Guarantee of Securities of Subsidiaries Guarantee of Securities of Subsidiaries Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Operating Revenues:
Transportation	$ 269.8	$ 256.9	$ 575.7	$ 546.5
Parking and lending	4.4	2.8	8.5	5.6
Storage	23.6	20.9	44.6	40.2
Other	8.5	18.0	22.5	36.0
Total operating revenues	306.3	298.6	651.3	628.3
Operating Costs and Expenses:
Fuel and transportation	11.6	24.8	31.9	52.9
Operation and maintenance	48.3	53.4	91.7	94.7
Administrative and general	35.5	31.8	70.2	62.4
Other operating costs and expenses	101.6	102.7	206.6	209.6
Total operating costs and expenses	197.0	212.7	400.4	419.6
Operating income	109.3	85.9	250.9	208.7
Other Deductions (Income):
Interest expense	45.4	45.9	88.0	91.1
Interest expense - affiliates, net	0.0	0.0	0.0	0.0
Interest income	(0.1)	(0.1)	(0.2)	(0.2)
Equity in earnings of subsidiaries	0.0	0.0	0.0	0.0
Miscellaneous other income, net	(1.9)	(0.4)	(4.0)	(0.6)
Total other (income) deductions	43.4	45.4	83.8	90.3
Income (loss) before income taxes	65.9	40.5	167.1	118.4
Income taxes	0.2	0.1	0.4	0.3
Net income (loss)	65.7	40.4	166.7	118.1
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	(0.2)
Other operating costs and expenses	0.1	0.2	0.2	0.2
Total operating costs and expenses	0.1	0.0	0.3	0.0
Operating income	(0.1)	0.0	(0.3)	0.0
Other Deductions (Income):
Interest expense	0.0	0.0	0.0	0.0
Interest income - affiliates, net	(9.0)	(7.2)	(17.9)	(13.9)
Interest income	0.0	0.0	0.0	0.0
Equity in earnings of subsidiaries	(57.0)	(33.2)	(149.3)	(104.2)
Miscellaneous other income, net	0.2	0.0	0.2	0.0
Total other (income) deductions	(65.8)	(40.4)	(167.0)	(118.1)
Income (loss) before income taxes	65.7	40.4	166.7	118.1
Income taxes	0.0	0.0	0.0	0.0
Net income (loss)	65.7	40.4	166.7	118.1
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 income	0.0	0.0	0.0	0.0
Other Deductions (Income):
Interest expense	30.4	27.9	56.5	51.8
Interest expense - affiliates, net	11.5	9.1	23.5	18.6
Interest income	0.0	0.0	0.0	0.0
Equity in earnings of subsidiaries	(98.9)	(70.2)	(229.3)	(174.6)
Miscellaneous other income, net	0.0	0.0	0.0	0.0
Total other (income) deductions	(57.0)	(33.2)	(149.3)	(104.2)
Income (loss) before income taxes	57.0	33.2	149.3	104.2
Income taxes	0.0	0.0	0.0	0.0
Net income (loss)	57.0	33.2	149.3	104.2
Reportable Legal Entities | Non-Guarantor Subsidiaries
Operating Revenues:
Transportation	291.7	278.6	619.1	590.2
Parking and lending	5.4	3.0	9.5	5.8
Storage	23.6	20.9	44.6	40.2
Other	8.5	18.0	22.5	36.0
Total operating revenues	329.2	320.5	695.7	672.2
Operating Costs and Expenses:
Fuel and transportation	34.5	46.7	76.3	96.8
Operation and maintenance	48.3	53.4	91.7	94.7
Administrative and general	35.5	32.0	70.1	62.6
Other operating costs and expenses	101.5	102.5	206.4	209.4
Total operating costs and expenses	219.8	234.6	444.5	463.5
Operating income	109.4	85.9	251.2	208.7
Other Deductions (Income):
Interest expense	15.0	18.0	31.5	39.3
Interest income - affiliates, net	(2.5)	(1.9)	(5.6)	(4.7)
Interest income	(0.1)	(0.1)	(0.2)	(0.2)
Equity in earnings of subsidiaries	0.0	0.0	0.0	0.0
Miscellaneous other income, net	(2.1)	(0.4)	(4.2)	(0.6)
Total other (income) deductions	10.3	15.6	21.5	33.8
Income (loss) before income taxes	99.1	70.3	229.7	174.9
Income taxes	0.2	0.1	0.4	0.3
Net income (loss)	98.9	70.2	229.3	174.6
Eliminations
Operating Revenues:
Transportation	(21.9)	(21.7)	(43.4)	(43.7)
Parking and lending	(1.0)	(0.2)	(1.0)	(0.2)
Storage	0.0	0.0	0.0	0.0
Other	0.0	0.0	0.0	0.0
Total operating revenues	(22.9)	(21.9)	(44.4)	(43.9)
Operating Costs and Expenses:
Fuel and transportation	(22.9)	(21.9)	(44.4)	(43.9)
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	(22.9)	(21.9)	(44.4)	(43.9)
Operating 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	155.9	103.4	378.6	278.8
Miscellaneous other income, net	0.0	0.0	0.0	0.0
Total other (income) deductions	155.9	103.4	378.6	278.8
Income (loss) before income taxes	(155.9)	(103.4)	(378.6)	(278.8)
Income taxes	0.0	0.0	0.0	0.0
Net income (loss)	$ (155.9)	$ (103.4)	$ (378.6)	$ (278.8)