Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	$ 813,841	$ 644,231
Less: Accumulated depreciation and amortization	(251,691)	(205,637)
Property and Equipment, net	562,150	438,594
Land [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	24,708	24,708
Building And Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	36,459	36,269
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	6,372	5,016
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	109,336	91,302
Furniture And Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	14,555	13,852
Transportation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	620,951	472,634
Construction In Progress [Member]
Property, Plant and Equipment [Line Items]
Property and Equipment, gross	$ 1,460	$ 450