Segment Information - Segment Financial Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Total sales	$ 1,111,680	$ 932,253	$ 2,167,857	$ 1,848,287
Income (loss) from operations	123,523	113,357	254,802	227,869
Interest expense and other, net	(29,749)	(16,628)	(47,447)	(31,675)
Income from operations before income taxes	93,774	96,729	207,355	196,194
Freight Segment
Segment Reporting Information [Line Items]
Total sales	412,258	344,828	791,812	692,774
Income from operations before income taxes	84,347	63,165	153,969	134,387
Transit Segment
Segment Reporting Information [Line Items]
Total sales	699,422	587,425	1,376,045	1,155,513
Income from operations before income taxes	57,975	59,050	126,059	108,025
Operating Segments | Freight Segment
Segment Reporting Information [Line Items]
Total sales	430,957	354,967	822,513	712,000
Income (loss) from operations	84,347	63,165	153,969	134,387
Interest expense and other, net	0	0	0	0
Operating Segments | Transit Segment
Segment Reporting Information [Line Items]
Total sales	702,406	593,498	1,382,918	1,167,272
Income (loss) from operations	57,975	59,050	126,059	108,025
Interest expense and other, net	0	0	0	0
Intersegment sales/(elimination)
Segment Reporting Information [Line Items]
Total sales	(21,683)	(16,212)	(37,574)	(30,985)
Intersegment sales/(elimination) | Freight Segment
Segment Reporting Information [Line Items]
Total sales	18,699	10,139	30,701	19,226
Intersegment sales/(elimination) | Transit Segment
Segment Reporting Information [Line Items]
Total sales	2,984	6,073	6,873	11,759
Corporate Activities and Elimination
Segment Reporting Information [Line Items]
Income (loss) from operations	(18,799)	(8,858)	(25,226)	(14,543)
Interest expense and other, net	(29,749)	(16,628)	(47,447)	(31,675)
Income from operations before income taxes	$ (48,548)	$ (25,486)	$ (72,673)	$ (46,218)