Property and Equipment - Summary of Property and Equipment (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Property Plant And Equipment Disclosure [Line Items]
Cost	$ 146,271	$ 144,410
Accumulated Depreciation	(60,316)	(58,839)
Net Book Value	85,955	85,571
Land and Land Improvements [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	11,474	11,474
Net Book Value	11,474	11,474
Buildings and Other Structures [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	8,376	8,376
Accumulated Depreciation	(4,081)	(3,893)
Net Book Value	4,295	4,483
Rail and Other Track Material [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	31,107	30,835
Accumulated Depreciation	(7,614)	(7,176)
Net Book Value	23,493	23,659
Ballast [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	5,922	5,885
Accumulated Depreciation	(1,814)	(1,737)
Net Book Value	4,108	4,148
Ties [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	51,297	50,917
Accumulated Depreciation	(25,392)	(24,537)
Net Book Value	25,905	26,380
Bridges and Trestles [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	8,996	7,918
Accumulated Depreciation	(2,802)	(2,648)
Net Book Value	6,194	5,270
Other [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	1,361	1,281
Accumulated Depreciation	(953)	(935)
Net Book Value	408	346
Property [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	118,533	116,686
Accumulated Depreciation	(42,656)	(40,926)
Net Book Value	75,877	75,760
Office [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	459	453
Accumulated Depreciation	(411)	(388)
Net Book Value	48	65
Locomotives [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	12,262	12,622
Accumulated Depreciation	(8,152)	(7,815)
Net Book Value	4,110	4,807
Railcars [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	2,707	2,374
Accumulated Depreciation	(895)	(1,650)
Net Book Value	1,812	724
Vehicles [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	2,952	2,718
Accumulated Depreciation	(2,271)	(2,153)
Net Book Value	681	565
Signals and Crossing [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	1,111	1,346
Accumulated Depreciation	(673)	(837)
Net Book Value	438	509
Track [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	2,349	2,265
Accumulated Depreciation	(1,502)	(1,416)
Net Book Value	847	849
Other [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	4,690	4,612
Accumulated Depreciation	(3,756)	(3,654)
Net Book Value	934	958
Equipment [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	26,530	26,390
Accumulated Depreciation	(17,660)	(17,913)
Net Book Value	8,870	8,477
Construction in Process [Member]
Property Plant And Equipment Disclosure [Line Items]
Cost	1,208	1,334
Net Book Value	$ 1,208	$ 1,334