Fair Value Measurements and Other Comprehensive Income (OCI) Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Beginning balance	$ (72.8)	$ (63.8)
Gain (loss) recorded in accumulated other comprehensive loss	0	(0.9)
Transportation operating revenues	289.6	304.3
Interest expense	45.2	40.9
Pension and other postretirement benefit costs	(2.1)	(1.9)
Ending balance	(74.3)	(65.7)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Transportation operating revenues		0.1
Other operating revenues		0.2
Interest expense	0.6	0.6
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	2.4
Beginning balance	(10.8)	(12.7)
Gain (loss) recorded in accumulated other comprehensive loss		(0.9)
Pension and other postretirement benefit costs	0	0
Ending balance	(10.2)	(12.7)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Transportation operating revenues		0.1
Other operating revenues		0.2
Interest expense	0.6	0.6
Accumulated Defined Benefit Plans Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Beginning balance	(62.0)	(51.1)
Gain (loss) recorded in accumulated other comprehensive loss		0
Pension and other postretirement benefit costs	(2.1)	(1.9)
Ending balance	(64.1)	(53.0)
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Transportation operating revenues		0
Other operating revenues		0
Interest expense	$ 0	$ 0