Segment Financial Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Total sales	$ 797,271	$ 631,398	$ 2,223,588	$ 1,884,910
Income (loss) from operations	135,977	109,871	390,146	326,092
Interest expense and other, net	(4,748)	(5,487)	(13,497)	(12,547)
Income from operations before income taxes	131,229	104,384	376,649	313,545
External Customers
Segment Reporting Information [Line Items]
Total sales	797,271	631,398	2,223,588	1,884,910
Intersegment Sales/(Elimination)
Segment Reporting Information [Line Items]
Total sales	0	0	0	0
Operating Segments | Freight Segment
Segment Reporting Information [Line Items]
Total sales	463,110	345,943	1,277,411	1,029,453
Income (loss) from operations	102,182	77,299	294,396	225,734
Interest expense and other, net	0	0	0	0
Income from operations before income taxes	102,182	77,299	294,396	225,734
Operating Segments | Transit Segment
Segment Reporting Information [Line Items]
Total sales	345,764	293,507	979,347	881,248
Income (loss) from operations	40,734	36,335	112,009	110,809
Interest expense and other, net	0	0	0	0
Income from operations before income taxes	40,734	36,335	112,009	110,809
Operating Segments | External Customers | Freight Segment
Segment Reporting Information [Line Items]
Total sales	453,329	340,533	1,250,337	1,009,069
Operating Segments | External Customers | Transit Segment
Segment Reporting Information [Line Items]
Total sales	343,942	290,865	973,251	875,841
Operating Segments | Intersegment Sales/(Elimination) | Freight Segment
Segment Reporting Information [Line Items]
Total sales	9,781	5,410	27,074	20,384
Operating Segments | Intersegment Sales/(Elimination) | Transit Segment
Segment Reporting Information [Line Items]
Total sales	1,822	2,642	6,096	5,407
Corporate Activities and Elimination
Segment Reporting Information [Line Items]
Total sales	(11,603)	(8,052)	(33,170)	(25,791)
Income (loss) from operations	(6,939)	(3,763)	(16,259)	(10,451)
Interest expense and other, net	(4,748)	(5,487)	(13,497)	(12,547)
Income from operations before income taxes	(11,687)	(9,250)	(29,756)	(22,998)
Corporate Activities and Elimination | External Customers
Segment Reporting Information [Line Items]
Total sales	0	0	0	0
Corporate Activities and Elimination | Intersegment Sales/(Elimination)
Segment Reporting Information [Line Items]
Total sales	$ (11,603)	$ (8,052)	$ (33,170)	$ (25,791)