Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Nonoperating Income, Residual Sharing Income									$ 10.2	$ 83.6	$ 13.4
Effective Income Tax Rate Reconciliation, Tax Credit, Foreign, Amount	$ 7.1								0.0	7.8	0.0
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property									52.5	52.9	99.7
State deferred tax rate change impact									5.0	0.0	14.1
Revaluation of deferred tax liabilities	315.9								(371.4)	0.0	0.0
Selected Quarterly Financial Data (unaudited)
Revenues	352.8	$ 359.6	$ 348.4	$ 316.1	$ 362.1	$ 362.9	$ 358.9	$ 334.4	1,376.9	1,418.3	1,449.9
Net income	$ 342.1	$ 49.0	$ 53.4	$ 57.5	$ 30.9	$ 95.7	$ 61.2	$ 69.3	$ 502.0	$ 257.1	$ 205.3
Share Data
Basic earnings per share (in dollars per share)	$ 8.98	$ 1.27	$ 1.37	$ 1.46	$ 0.78	$ 2.39	$ 1.51	$ 1.67	$ 12.95	$ 6.35	$ 4.76
Diluted earnings per share (in dollars per share)	$ 8.83	$ 1.25	$ 1.35	$ 1.44	$ 0.77	$ 2.36	$ 1.49	$ 1.66	$ 12.75	$ 6.29	$ 4.69
Portfolio Management [Member]
Nonoperating Income, Residual Sharing Income		$ 30.3							$ 9.6	$ 82.8	$ 12.6
Selected Quarterly Financial Data (unaudited)
Revenues									29.9	56.6	92.5
Rail North America [Member]
Nonoperating Income, Residual Sharing Income									0.6	0.8	0.8
Asset Impairment Charges	$ 19.2
Selected Quarterly Financial Data (unaudited)
Revenues									977.4	1,018.5	$ 1,006.8
Foreign Tax Authority [Member]
Equity Method Investment, Summarized Financial Information, Income Tax		$ 3.9							$ 3.9	$ 7.7