Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Numerator:
Net income	$ 30.9		$ 95.7		$ 61.2		$ 69.3		$ 58.2		$ 39.5		$ 45.4		$ 62.2		$ 257.1	$ 205.3		$ 205.0
Denominator:
Denominator for basic earnings per share — weighted average shares																	40.5	43.1		45.0
Effect of dilutive securities:
Equity compensation plans																	0.4	0.7		0.8
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion																	40.9	43.8		45.8
Basic earnings per share (in dollars per share)	$ 0.78	[1]	$ 2.39	[1]	$ 1.51	[1]	$ 1.67	[1]	$ 1.38	[1]	$ 0.92	[1]	$ 1.04	[1]	$ 1.41	[1]	$ 6.35	$ 4.76	[1]	$ 4.55
Diluted earnings per share (in dollars per share)	$ 0.77	[1]	$ 2.36	[1]	$ 1.49	[1]	$ 1.66	[1]	$ 1.37	[1]	$ 0.91	[1]	$ 1.03	[1]	$ 1.39	[1]	$ 6.29	$ 4.69	[1]	$ 4.48

[1] (1)Quarterly earnings per share may not be additive, as per share amounts are computed independently for each quarter and the full year is based on the respective weighted average common shares and common stock equivalents outstanding.(2)In the third quarter of 2016, net income included $30.3 million of income related to the settlement of a residual sharing agreement and $3.9 million of benefit resulting from a reduction in the statutory income tax rate in the UK. In the third quarter of 2015, net income included $26.6 million of net loss related to the exit of marine investments at Portfolio Management.(3)In the fourth quarter of 2016, net income included $19.2 million of impairment losses related to certain railcars at Rail North America and $7.1 million of benefit related to the utilization of foreign tax credits. In the fourth quarter of 2015, net income included $9.0 million of net gain related to the exit of marine investments at Portfolio Management. In addition, net income included $14.1 million of expense attributable to an increase in our effective state income tax rate, $7.7 million of benefit resulting from a reduction in the statutory income tax rate in the UK, and $5.6 million of net expenses associated with an early retirement program offered to certain employees.