Segment Information - Segment Financial Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Total sales	$ 1,906.9	$ 1,865.1	$ 5,749.4	$ 5,532.4
Income (loss) from operations	217.0	206.7	611.9	583.4
Interest expense and other, net	(41.7)	(31.3)	(109.7)	(144.5)
Income (loss) before income taxes	175.3	175.4	502.2	438.9
Freight Segment
Segment Reporting Information [Line Items]
Total sales	1,295.0	1,237.3	3,814.2	3,743.0
Transit Segment
Segment Reporting Information [Line Items]
Total sales	611.9	627.8	1,935.2	1,789.4
Operating Segments | Freight Segment
Segment Reporting Information [Line Items]
Total sales	1,307.3	1,248.2	3,850.7	3,779.3
Income (loss) from operations	195.2	160.2	510.2	463.4
Interest expense and other, net	0.0	0.0	0.0	0.0
Income (loss) before income taxes	195.2	160.2	510.2	463.4
Operating Segments | Transit Segment
Segment Reporting Information [Line Items]
Total sales	620.5	635.6	1,960.5	1,812.4
Income (loss) from operations	43.9	64.1	159.3	172.9
Interest expense and other, net	0.0	0.0	0.0	0.0
Income (loss) before income taxes	43.9	64.1	159.3	172.9
Corporate Activities and Elimination
Segment Reporting Information [Line Items]
Total sales	(20.9)	(18.7)	(61.8)	(59.3)
Income (loss) from operations	(22.1)	(17.6)	(57.6)	(52.9)
Interest expense and other, net	(41.7)	(31.3)	(109.7)	(144.5)
Income (loss) before income taxes	(63.8)	(48.9)	(167.3)	(197.4)
Corporate Activities and Elimination | Freight Segment
Segment Reporting Information [Line Items]
Total sales	12.3	10.9	36.5	36.3
Corporate Activities and Elimination | Transit Segment
Segment Reporting Information [Line Items]
Total sales	$ 8.6	$ 7.8	$ 25.3	$ 23.0