SEGMENT INFORMATION - Sales by Product (Details) - USD ($) $ 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
Revenue from External Customer [Line Items]
Total sales	$ 2,368.4	$ 2,001.7	$ 2,236.3	$ 1,593.6	$ 1,117.8	$ 1,077.8	$ 1,111.7	$ 1,056.2	$ 8,200.0	$ 4,363.5	$ 3,881.7
Freight Segment
Revenue from External Customer [Line Items]
Total sales									5,441.4	1,766.4	1,538.6
Transit Segment
Revenue from External Customer [Line Items]
Total sales									2,758.6	2,597.1	2,343.1
Equipment | Freight Segment
Revenue from External Customer [Line Items]
Total sales									1,699.7	0.0	0.0
Components | Freight Segment
Revenue from External Customer [Line Items]
Total sales									1,073.5	1,169.1	1,004.6
Digital Electronics | Freight Segment
Revenue from External Customer [Line Items]
Total sales									677.1	474.1	400.8
Services
Revenue from External Customer [Line Items]
Total sales									1,292.1	185.5	196.1
Services | Freight Segment
Revenue from External Customer [Line Items]
Total sales									1,991.1	123.2	133.2
Original Equipment Manufacturer | Transit Segment
Revenue from External Customer [Line Items]
Total sales									1,286.6	1,194.0	1,050.0
Aftermarket | Transit Segment
Revenue from External Customer [Line Items]
Total sales									$ 1,472.0	$ 1,403.1	$ 1,293.1