Leases - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2016	Dec. 31, 2015
Operating Leased Assets [Line Items]
Sublease income	$ 1,816	$ 208
Rent expense for all operating leases	24,116	22,542	21,452
Scenario, Forecast | Railroad Equipment
Operating Leased Assets [Line Items]
Sublease income				$ 2,242	$ 4,007