Leases

We lease our corporate office facilities in San Antonio, Texas at a payment escalating from $35,694 per month in January 2013 to $42,635 per month in December 2020 pursuant to a lease which extends through December 2020, but which is cancelable as early as December 2016 with applicable penalties. We recognize rent expense on a straight-line basis for our corporate office lease. In addition, we lease real estate at 54 other locations under non-cancelable operating leases with payments ranging from $380 per month to $36,000 per month, pursuant to leases expiring through May 2022. These real estate locations are used primarily for field offices and storage and maintenance yards. We also lease vehicles, office and other equipment under non-cancelable operating leases expiring through December 2016.

Future lease obligations required under non-cancelable operating leases as of December 31, 2012 were as follows (amounts in thousands):



Year ended December 31,
2013	$	4,751
2014	3,890
2015	2,268
2016	1,436
2017	1,058
Thereafter	2,976
	$	16,379

Rent expense under operating leases for the years ended December 31, 2012, 2011 and 2010 was $5.6 million, $3.6 million and $2.9 million, respectively.