Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2015
Asset Impairment Charges									$ 6.6	$ 9.0	$ 8.6
Nonoperating Income, Residual Sharing Income									1.9	2.5	10.2
Effective Income Tax Rate Reconciliation, Tax Credit, Foreign, Amount									0.0	1.4	0.0
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property									66.7	79.4	52.5
State deferred tax rate change impact									0.0	0.0	5.0
Revaluation of deferred tax liabilities					$ (17.9)				0.0	(9.4)	371.4
Selected Quarterly Financial Data (unaudited)
Revenues	$ 356.7	$ 360.7	$ 359.4	$ 317.0	356.4	$ 349.7	$ 349.5	$ 305.3	1,393.8	1,360.9	1,376.9
Net income	$ 56.6	$ 45.1	$ 68.0	$ 41.5	$ 49.2	$ 47.0	$ 38.8	$ 76.3	$ 211.2	$ 211.3	$ 502.0
Share Data
Basic earnings per share (in dollars per share)	$ 1.62	$ 1.28	$ 1.89	$ 1.14	$ 1.32	$ 1.25	$ 1.03	$ 2.02	$ 5.92	$ 5.62	$ 12.95
Diluted earnings per share (in dollars per share)	$ 1.59	$ 1.25	$ 1.86	$ 1.12	$ 1.30	$ 1.22	$ 1.01	$ 1.98	$ 5.81	$ 5.52	$ 12.75
Rail International [Member]
Asset Impairment Charges									$ 0.0	$ 3.9	$ 0.3
Nonoperating Income, Residual Sharing Income									0.0	0.0	0.0
Selected Quarterly Financial Data (unaudited)
Revenues									227.7	217.5	197.1
Portfolio Management [Member]
Asset Impairment Charges									6.2	4.5	3.7
Nonoperating Income, Residual Sharing Income									1.5	1.1	9.6
Selected Quarterly Financial Data (unaudited)
Revenues									9.9	16.1	29.9
Rail North America [Member]
Asset Impairment Charges									0.4	0.6	4.6
Nonoperating Income, Residual Sharing Income									0.4	1.4	0.6
Selected Quarterly Financial Data (unaudited)
Revenues									964.5	$ 941.5	$ 977.4
Canada Revenue Agency [Member]
Revaluation of deferred tax liabilities									$ 2.8
Cardinal Marine [Domain]
Noncontrolling Interest, Ownership Percentage by Parent												50.00%