GATX Corporation - Additional Information (Detail) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Sep. 30, 2014 USD ($) Millgondola	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) Millgondola Automobile $ / shares shares	Dec. 31, 2014 USD ($) Millgondola $ / shares shares	Dec. 31, 2013	Dec. 31, 2012 Millgondola Railcar	Jun. 30, 2014 Opentop_Hoppers	Jan. 10, 2008 USD ($) Millgondola Opentop_Hoppers $ / shares shares
Lease Commitments [Line Items]
Number of shares acquired by GATX | shares			4,862,693	4,859,871				239,523
Percentage of shares acquired by GATX								4.99%
Cost of newly-issued shares common stock			$ 2,432	$ 2,430				$ 5,500
Cost per share of newly-issued shares common stock | $ / shares			$ 0.50	$ 0.50				$ 23
Number of mill gondolas | Millgondola	52			10				72
Number of open-top hoppers | Opentop_Hoppers							75	137
Number of gondolas returned | Millgondola						20
Lease expiration term			7 years
Number of auto carrying railcars | Railcar						200
Lease expiration date			Dec. 31, 2025		Dec. 31, 2023
Number of automobile carrying railcars | Automobile			76
Lease renewal term			1 year
Operating lease maturity period			Mar. 01, 2014
Number of gondolas entered into per diem lease | Millgondola			8
GATX [Member]
Lease Commitments [Line Items]
Car-hire earned from other railroads and remitted to GATX			$ 3,700	$ 3,700
Gondolas [Member]
Lease Commitments [Line Items]
Period of operating leases			7 years
Annual rent adjusted				163
Annual rental maturity date	2019-12		2015-01
Annual lease rate	$ 183
Rental expense			$ 183	$ 163
Lease expiration extended date			2019-12
Auto Carrying Railcars [Member]
Lease Commitments [Line Items]
Lease expiration date			Dec. 31, 2014
Locomotives [Member]
Lease Commitments [Line Items]
Cost of lease six-axle EMD SD-60 locomotives		$ 233
Minimum [Member] | Gondolas [Member]
Lease Commitments [Line Items]
Annual rent			$ 248