Revolving Line of Credit - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Oct. 31, 2011	Aug. 02, 2007
Line of Credit Facility [Line Items]
Borrowing capacity under revolving line of credit	$ 150	$ 150	$ 100	$ 30
Further increases in total commitment		175
Amount outstanding on line of credit	70.8	70.4
Available on revolving line of credit	79.2
Maturity date of the facility	Dec. 21, 2016
Qualified lease receivables eligible under the borrowing base computation	128.7
London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Amount outstanding on line of credit	65.0
Base Rate [Member]
Line of Credit Facility [Line Items]
Amount outstanding on line of credit	5.8
Minimum [Member]
Line of Credit Facility [Line Items]
Borrowing capacity under revolving line of credit		100
Interest rate on loans	2.80%
Maximum [Member]
Line of Credit Facility [Line Items]
Borrowing capacity under revolving line of credit		$ 150
Interest rate on loans	4.00%