Amtrak Agreement - Schedule of Remaining Track Mileage Credit (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Agreement With Related Party [Abstract]
Mileage credit available, beginning	$ 1,115	$ 1,890	$ 1,200	$ 1,994
Utilized	210	201	295	305
Mileage credit available, ending	$ 905	$ 1,689	$ 905	$ 1,689