Business Segment Information - Financial Information Regarding Company's Operations by Geographic Area (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		$ 1,854,262	$ 1,545,132
Net revenues	[1]	635,838	527,605
Operating income		192,818	146,114
Identifiable assets at period end		3,212,481	2,902,719	$ 3,117,008
Capital expenditures		13,796	12,761
Depreciation and amortization		13,922	11,927
Equity		1,982,266	1,951,720	$ 1,994,373	$ 1,847,213
United States Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		534,869	426,019
Other North America Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		76,525	59,899
Latin America Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		32,722	22,103
North Asia Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		630,572	566,428
South Asia Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		168,600	147,240
Europe Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		305,792	231,457
Middle East Africa And India Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		105,182	91,986
Geography Eliminations | United States Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		28,881	24,313
Geography Eliminations | Other North America Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		3,562	2,639
Geography Eliminations | Latin America Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		3,387	3,635
Geography Eliminations | North Asia Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		6,017	5,051
Geography Eliminations | South Asia Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		6,550	5,431
Geography Eliminations | Europe Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		14,188	9,322
Geography Eliminations | Middle East Africa And India Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		5,355	4,964
Operating Segments | United States Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		563,750	450,332
Net revenues	[1]	277,185	230,785
Operating income		69,648	52,346
Identifiable assets at period end		1,558,181	1,536,520
Capital expenditures		3,171	5,242
Depreciation and amortization		8,765	7,753
Equity		1,247,522	1,192,601
Operating Segments | Other North America Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		80,087	62,538
Net revenues	[1]	30,447	25,793
Operating income		9,018	5,051
Identifiable assets at period end		165,368	106,068
Capital expenditures		1,528	234
Depreciation and amortization		398	372
Equity		68,050	51,812
Operating Segments | Latin America Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		36,109	25,738
Net revenues	[1]	14,745	14,916
Operating income		2,370	3,451
Identifiable assets at period end		55,036	50,344
Capital expenditures		505	255
Depreciation and amortization		368	320
Equity		27,226	27,248
Operating Segments | North Asia Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		636,589	571,479
Net revenues	[1]	137,208	111,833
Operating income		69,923	53,352
Identifiable assets at period end		542,965	514,509
Capital expenditures		784	1,240
Depreciation and amortization		1,376	1,320
Equity		278,651	371,152
Operating Segments | South Asia Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		175,150	152,671
Net revenues	[1]	44,406	37,995
Operating income		15,132	13,224
Identifiable assets at period end		153,411	122,765
Capital expenditures		329	373
Depreciation and amortization		583	531
Equity		105,642	105,726
Operating Segments | Europe Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		319,980	240,779
Net revenues	[1]	98,635	75,958
Operating income		19,278	11,646
Identifiable assets at period end		533,012	379,853
Capital expenditures		7,191	5,078
Depreciation and amortization		1,976	1,171
Equity		160,038	118,211
Operating Segments | Middle East Africa And India Segment
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		110,537	96,950
Net revenues	[1]	33,201	29,731
Operating income		7,438	7,043
Identifiable assets at period end		208,173	188,098
Capital expenditures		288	339
Depreciation and amortization		456	460
Equity		128,487	119,627
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		(67,940)	(55,355)
Net revenues	[1]	11	594
Operating income		11	1
Identifiable assets at period end		(3,665)	4,562
Equity		$ (33,350)	$ (34,657)

[1] Net revenues are a non-GAAP measure calculated as revenues less directly related operating expenses attributable to the Company's principal services. The Company's management believes that net revenues are a better measure than total revenues when evaluating the Company's operating segment performance since total revenues earned as a freight consolidator include the carriers' charges for carrying the shipment, whereas revenues earned in other capacities include primarily the commissions and fees earned by the Company. Net revenue is one of the Company's primary operational and financial measures and demonstrates the Company's ability to concentrate and leverage purchasing power through effective consolidation of shipments from customers utilizing a variety of transportation carriers and optimal routings.