SEGMENT INFROMATION - Segment Financial Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Total sales	$ 832,849	$ 809,527	$ 847,028	$ 818,594	$ 820,866	$ 797,271	$ 731,068	$ 695,249	$ 3,307,998	$ 3,044,454	$ 2,566,392
Income (loss) from operations	151,209	$ 152,078	$ 155,860	$ 148,420	136,963	$ 135,977	$ 132,323	$ 121,846	607,567	527,109	437,310
Interest expense and other, net									(22,199)	(19,254)	(16,223)
Income (loss) from operations before income taxes									585,368	507,855	421,087
Income (loss) from operations before income taxes									585,368	507,855	421,087
Depreciation and amortization									64,734	61,261	51,193
Capital expenditures									49,428	47,662	41,238
Segment assets	3,300,335				3,303,841				3,300,335	3,303,841	2,821,997
External Customers
Segment Reporting Information [Line Items]
Total sales									3,307,998	3,044,454	2,566,392
Operating Segments | Freight Segment
Segment Reporting Information [Line Items]
Total sales									2,090,087	1,767,662	1,423,566
Income (loss) from operations									482,640	402,456	309,133
Income (loss) from operations before income taxes											309,133
Income (loss) from operations before income taxes									482,640	402,456
Depreciation and amortization									36,834	34,579	30,645
Capital expenditures									24,715	22,913	22,020
Segment assets	2,766,798				2,516,645				2,766,798	2,516,645	2,258,773
Operating Segments | Transit Segment
Segment Reporting Information [Line Items]
Total sales									1,264,178	1,320,335	1,175,281
Income (loss) from operations									150,988	147,821	143,634
Income (loss) from operations before income taxes											143,634
Income (loss) from operations before income taxes									150,988	147,821
Depreciation and amortization									26,196	24,956	19,103
Capital expenditures									22,996	22,859	17,119
Segment assets	2,221,027				2,024,312				2,221,027	2,024,312	1,706,829
Operating Segments | External Customers | Freight Segment
Segment Reporting Information [Line Items]
Total sales									2,054,715	1,731,477	1,398,103
Operating Segments | External Customers | Transit Segment
Segment Reporting Information [Line Items]
Total sales									1,253,283	1,312,977	1,168,289
Operating Segments | Intersegment Sales/(Elimination) | Freight Segment
Segment Reporting Information [Line Items]
Total sales									35,372	36,185	25,463
Operating Segments | Intersegment Sales/(Elimination) | Transit Segment
Segment Reporting Information [Line Items]
Total sales									10,895	7,358	6,992
Corporate Activities and Elimination
Segment Reporting Information [Line Items]
Total sales									(46,267)	(43,543)	(32,455)
Income (loss) from operations									(26,061)	(23,168)	(15,457)
Interest expense and other, net									22,199	(19,254)	(16,223)
Income (loss) from operations before income taxes											(31,680)
Income (loss) from operations before income taxes									(48,260)	(42,422)
Depreciation and amortization									1,704	1,726	1,445
Capital expenditures									1,717	1,890	2,099
Segment assets	$ (1,687,490)				$ (1,237,116)				(1,687,490)	(1,237,116)	(1,143,605)
Corporate Activities and Elimination | Intersegment Sales/(Elimination)
Segment Reporting Information [Line Items]
Total sales									$ (46,267)	$ (43,543)	$ (32,455)