Segment Information - Sales by Product (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Total sales	$ 2,012.3	$ 1,737.4	$ 3,842.5	$ 3,667.3
Freight Segment
Segment Reporting Information [Line Items]
Total sales	1,335.9	1,204.7	2,519.2	2,505.7
Transit Segment
Segment Reporting Information [Line Items]
Total sales	676.4	532.7	1,323.3	1,161.6
Equipment | Freight Segment
Segment Reporting Information [Line Items]
Total sales	327.8	334.9	589.6	742.9
Components | Freight Segment
Segment Reporting Information [Line Items]
Total sales	224.3	195.5	426.8	415.9
Digital Electronics | Freight Segment
Segment Reporting Information [Line Items]
Total sales	161.8	165.5	318.3	339.1
Services
Segment Reporting Information [Line Items]
Total sales	424.3	331.3	769.4	670.4
Services | Freight Segment
Segment Reporting Information [Line Items]
Total sales	622.0	508.8	1,184.5	1,007.8
Original Equipment Manufacturer | Transit Segment
Segment Reporting Information [Line Items]
Total sales	320.4	227.3	607.2	514.3
Aftermarket | Transit Segment
Segment Reporting Information [Line Items]
Total sales	$ 356.0	$ 305.4	$ 716.1	$ 647.3