Description of Business and Summary of Significant Accounting Policies - Depreciation Provided Using Straight-line Method over Estimated useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2014
Ties [Member]
Track structure:
Estimated useful life of assets	40 years
Rail and Other Track Material [Member]
Track structure:
Estimated useful life of assets	67 years
Ballast [Member]
Track structure:
Estimated useful life of assets	67 years
Bridges and Trestles [Member]
Track structure:
Estimated useful life of assets	67 years
Other [Member]
Track structure:
Estimated useful life of assets	33 years
Buildings and Other Structures [Member] | Minimum [Member]
Track structure:
Estimated useful life of assets	33 years
Buildings and Other Structures [Member] | Maximum [Member]
Track structure:
Estimated useful life of assets	45 years
Equipment, Including Rolling Stock [Member] | Minimum [Member]
Track structure:
Estimated useful life of assets	4 years
Equipment, Including Rolling Stock [Member] | Maximum [Member]
Track structure:
Estimated useful life of assets	25 years